I have now been studying Japanese for seven years (time flies XD) I started to want to learn Japanese, because I knew I needed to find a way out of the mandatory French and Spanish, and I wanted to go with something that not everyone else in the school knew XD My path to how I am today is kind of long though lol XD
1) Everything started with a small dictionary and a basics book (can't even remember the name, but it got me interested and that is really the first step)
2) Then I took a Japanese class at school, and used a book called Yokoso (In my personal opinion, this is not a very good self-study book, only really useful in a classroom setting)
3) From there I moved schools, and with that changed books (Japanese for busy people) I enjoyed this book, and really got most of my true foundation from this series, I think I finished the first and second book.
4) When I started (3), I got the idea to try to emmerse myself as much as possible. My way of doing that, was buying manga in Japanese. My series of choice was "One Piece." Keep in mind that this is a huge jump, and some people can't do it because it's very tedious. I went from not being able to do more than read most of the book when I first started, and 50 books later, can finish a book off pretty quickly with high knowledge of what happened. My point is that it takes effort at first stumbling from page to page, and you need to set goals, but if you keep at it, there is a LOT you can pick up from reading.
5) Around the same time as (4) I started cramming daily at kanji. I bought a kanji book (it really doesn't matter what kind) and picked out a kanji every day and wrote it on a flashcard. Once a day I would add to my pile. Practicing every single day at school over and over again (the entire pile) Somewhere along the line I started to speed it up past 1 flashcard per day, and eventually it ended up at 5 per day. (Kanji is really not something to take lightly, if you know kanji, you can really get the meaning of a word by just looking at the kanji for it, it's VERY helpful and unfortunately most people don't care enough to stick with kanji) If you really get a good foundation in Kanji, you can look at a word you may have never seen before, and sometimes be able to make an accurate evaluation in your mind what it means. It happens :P
6) Got a private tutor. She emphasized Kanji... no joke, it's important lol...
7) Somewhere during this time, I started getting into music. Find a song you like, go online and try to read the lyrics in Japanese as they sing it. Not only does it help your reading flow, when you do go to Japan eventually, you will be building a Karaoke arsenal (it's a fun and very good way to interact with Japanese people that are your age)
8) Started watching Anime. This is gradual, and it is very hard to moderate yourself. At first I watched it with subtitles on. Then with due time I realized that I was doing more reading than listening (even though I was listening a little bit) That is when I decided to move to watching it with subtitles, and then subtitleless so that I could focus on words. Eventually it lead to no subtitles at all. (If you are watching a Japanese movie with Japanese subtitles, it's fun to watch it through without any subtitles at all, then turn on the Japanese subtitles for a second time through so that you can ensure how much you comprehended)
9) Bought an electronic dictionary. OMG does this help. The problem is that they cost quite a bit of money. If you have one though the amount of learning that you can do is amazing. If you are going to read any manga as I had said at an earlier step, this really helps get you started. When I first got the dictionary I was using it all the time, but today I barely ever use it at all. You have to wait on it, it's a pretty slow, but worth-while progression. The downside to the electronic dictionary is that if you use it for too long, you will get sleepy!! It has that effect on you lol... kind of like information overload XD
10) Now this one depends on where you live. I live in America, so I did some research and found places near me, but for other people in other countries, you're going to need to do some research to figure this one out. I looked-up, and participated in the Japanese Language Proficiency test. I live in America, and the closest locations to me are either Chicago or New York. Both of these locations hold the test, which is administered once a year. 4th level is the lowest, and 1st is the higest level. I started off taking the level 3 test, so I do not know the compexity of the level 4 test very well. I would say that a person who graduates high school level Japanese would be able to take and pass level 4. If they are a really good student, level 3. As far as college goes, there is such a wide range that it is hard to put a general idea on it. I think that most of the people in my class (in college) may be able to pass level 3 of the test. There is a huge ability gap between level 3 and level 2. Level 2 is a lot more difficult than level 3. I have heard that level 1 is a lot harder than level 2 as well. To this day, I have only taken level 3 and level 2, so that is the only area I really know well by personal experience. There are websites online that can prepaire you for each level. The point is that it is a good way to evaluate where you are, and where you need to imporve.
11) I started an internet Learn Japanese! thread on a website... it lasted for about a year-ish, but I quit when I got out of highschool. The good thing about doing something this drastic, is that you learn as you post updates. The more you work with the information and try to explain it to others, you will find that you learn more and more about it. It's a fun way to give back to the community, and help yourself at the same time.
12) Went to Japan. This is only good if you know how to use it. Unfortunately I cannot say what are good methods of learning while you are in Japan, it differs from person to person. Just know that the sky is the limit. You just have to make sure that you make the best use of your surroundings. The first time I went, I would use my electronic dictionary to read signs, which helps a lot. As of today, I have gone to Japan four times, and regardless of others that say going to Japan does not help learn the language, I bring something back with me every time I go.
13) This is the whole reason that I am on a Pokemon website lol XD I started picking up video games in the foreign language. It is an extention of reading Manga and watching Anime. Being able to play out a story on a game is difficult, because sometimes you need to know important information. It is a good way to help navigate you along the path of understanding the language by using it all the time. Even though I had previously bought Pokemon Blue in English when I was still in elementary school, I bought FireRed and Emerald when I went to Japan for my 3rd visit, and do not regret it one bit.
13) Going hand-in-hand with (12) I did a summer homestay for a month and about a week. It is a really fun way to get to know some culture and have fun. Host families genually care greatly for you, and want to see you happy. As long as you try to communicate with them, no matter how much you are embarassed to use your Japanese with them (it happens a lot) they are a great tool for you to get used to speaking Japanese. They really do want to help in any way they can.
14) After homestay, I moved onto novels in Japanese. That means no furigana XD They are difficult, but once you get the flow, you get used to them and they get interesting.
15) Just recently I have started getting into translating. It is hard sometimes because you know that there is a better way to translate what you are trying to, but you just can't figure out how you want to say it. It is good experience though. I started off using my translator quite a bit to confirm information before I finilized it as a translation, but you it starts to get easier, and quicker the more you try.
16) Almost as recent as (15), I eventually moved from watching Japanese movies with stories, to not only watching movies with stories, but also Japanese comedy shows. These are indeed harder than Anime and full-length movies. Aside from the sheer speed of talking, it gets difficult to actually get the joke XD Also, there are limited visuals, therefore you can not always get a clear visual image of what they are talking about sometimes outside of the words themselves. Again, very hard, but it helps stimulate you mentally because you have to move from putting what you see to what they are saying, to only listening to what is being said.
17) Though this is yet to happen, I will be doing a study abroad for one year starting in August. This is just one of the steps in the learning process, and I think that it will help a lot!
18) Use it as much as possible. When I first started, I never spoke it, just read it and wrote it. That really screwed me over later on down the line because I had no confidence in speaking, which was a huge hinderance. My teacher at school now (btw we are using Japanese the Spoken Language as a text book, which personally I do not reccomend for self-studying) is notorious for calling people on their cellphones and forcing people to call their classmates and make them hold a phone converstaion in Japanese (classmate to classmate is usually "WHY AREN'T YOU IN CLASS!?!") I used to be scared to death about talking on the phone in Japanese, but with practice you get used to it. Again the problem that you get with the phone is static interference and such where you can't always pick out every word they say. With practice it gets easier even though all of the limitations of the phone.
Well, believe it or not, that is seven years of my life jam-packed into 18 points lol... if there is one part of advice I can give, it's don't give up. I almost was ready to give up at one point, but I just took a break and then re-looked at it in the morning. Though it may feel that you are getitng nowhere at times, just don't give up! The process takes a while!! I hope that this has somewhat helped ^^