Okay, more lesson for everbody!
If you've got hiragana down then it's time to go over katakana.
Katakana
Hiragana and katakana are the two (mostly) phonetic syllabaries of Japanese. Although written differently they parallel each other in a similar way to English's capital and lower-case letters. So for us "JAPANESE" would be pronounced the same way as "japanese" and it's the same idea with hiragana and katakana.
Let's take the first row of hiragana as an example:
Which is katakana is written
But pronounced exactly the same. There's not much similarity with these five, is there? A few katakana are nearly identical to their hiragana counterparts, but for the most part you'll need to remember completely new kana. Here's a comparison of the two with the hiragana equivalents in parentheses:
I should say that according to most people who study Japanese (me included), katakana is harder to learn than hiragana. Or rather, it's harder to read. So take your time going over them because it's easier to get confused by them. In particular be careful for:ス (su) which looks quite similar to ヌ (nu) except that with ヌ the two lines cross.
ク (ku) and ケ (ke). On ケ the horizontal line extends out to the right further.
マ (ma) and ム (mu)
ワ (wa) and ウ (u), and also フ (fu), ラ (ra) and ヲ (wo)
チ(chi) and テ (te). チ has angled parallel lines and the curved line touches both of them. テ has horizontal lines and the curved line does not reach the top line.
Also be especially careful with:
ツ(tsu) and シ (shi). They look very similar. The main difference is that with ツ the two lines are more vertical and with シ the lines are more horizontal. This is one of the instances where it's very beneficial to know the stroke order of a kana, meaning how exactly it's supposed to be written.
ソ (so) and ン

. It's the same situation as above. ソ is more vertical and ン is more horizontal.
Katakana follows the same general rules as hiragana for making combined sounds. So, if you wanted to write "kyo" (きょ) in katakana you would still make the katakana "yo" (ヨ) smaller (ョ) and end up with キョ.
Something New
There's also something fairly common to katakana and not very common with hiragana (so much so that I didn't bother mentioning it before). In Japanese every kana ends with a vowel sound (except ン/ん). Each one of these can be extended to create a long vowel sound. For that we use this character: ー
As you can see, it's a line. What a long vowel sound actually means is a little more complicated. You'll recall that Japanese is composed of morae (usually a combination of a vowel and consonant) and each mora is like a syllable. It's a little difficult to explain so I'll just use examples as I go along.
Let's start with the kana と (to). Now let's add to it the kana う (u). What we get then is とう (tou). In effect, what we have done is extend the sound と makes by adding a second kana. When spoken とう should have a noticeably longer sound than と by itself. You don't change the sound you make; you simply hold the sound a little longer. You also (in normal speaking) don't try to make them two separate sounds. In English we would still consider とう a single syllable, just like we would for と alone.
Each of the five vowel sounds has a particular way of extending its sound.
For "a" sounds (ka, sa, etc.) you add あ (a).
For "i" sounds (ki, shi, etc.) you add い (i).
For "u" sounds (ku, su, etc.) you add う (u).
For "e" sounds (ke, se, etc.) you add い (i), not え (e).
For "o" sounds (ko, so, etc.) you add う (u), not お (o).
What you end up with are the sounds "_aa", "_ii", "_uu", "_ei", and "_ou" . (The underline represents the space for whatever consonant sound you're working with.) "_ei" and "_ou" are the most common of these sounds in native Japanese words, but all are common with loan words.
Now back to that line I introduced. The examples I gave were all in hiragana, but something different happens when we're using katakana. Instead of using a specific kana (ア, イ, ウ, エ, or オ) you simply add ー. So then カー would be "kaa", キー would be "kii", クー would be "kuu", ケー would be "kei", and コー would be "kou".
Several Things to Note
Using a line (ー) rather than a kana isn't always how you'll see these extended vowels. You are more or less as likely to see something written like コー as you are like コウ and they would be pronounced the same way and transliterated the same way. ー is more likely to appear in loan words, which are almost always written in katakana, than in native words which are written in katakana.
The transliterations of extended vowel sounds are sometimes written with a macron over the vowel (kō) rather than with an extra letter (kou). Sometimes you'll see different accent marks used, but it can be confusing as there isn't always a standard of what marks mean. Sometimes you'll see the word 'anime' written as 'animé' in which case the accent would mostly likely stand for a pronunciation mark and not to indicate that you are meant to lengthen the "e" sound.
Katakana is known for being the syllabary for writing loan, or foreign words. This is true, but it is not limited to that function. Among other things it is what Pokémon names are written with.
Quiz Yourself
1. [FONT="]ピカチュウ[/FONT]
2. [FONT="]ヤドン[/FONT]
3. [FONT="]エネコ[/FONT]
4. [FONT="]ルギア[/FONT]
5. [FONT="]フシギダネ[/FONT]
6. [FONT="]ストライク[/FONT]
7. [FONT="]ニドリーナ[/FONT]
8. [FONT="]リオル[/FONT]
9. [FONT="]チャーレム[/FONT]
10. [FONT="]アンノーン[/FONT]