Re: The Basics
The fact that one needs to have a theme is not necessarily true; however it is highly suggested that if one does not know what one is doing to play a deck revolving around a certain strategy. Goodstuff.dek still exists, though, and is actually remarkably effective when it is played (though not necessarily as effective as a streamlined singlepurpose deck).
Dueling Network is an excellent website, as Fabio has mentioned.
Staples should /probably/ be in every single deck that is built if you have space and they do not conflict with the core strategy of the deck; as well,you should not able to play cards that outclass them. If any of the above three are untrue, then staples are not in fact must-haves for one's deck (of course, Dark Hole and Monster Reborn are blatantly overpowered, so... make space for that kind of thing).
Tour Guide is not a staple by any sense of the word; it is a powerful card but not every deck needs it. It is highly splashable and very good, which is something entirely different. Effect Veiler would probably be more of a staple than Tour Guide.
Trap Dustshoot is a staple that is missing. Torrential Tribute is still (probably) a staple.
In fact, the only cards I would advocate running in every deck are the trinity of Limited Spell Cards (Storm / Hole / Reborn) and Trap Dustshoot. The other ones are all situational to varying degrees.
Extra Deck staples are fine-ish; depending on the deck's availability of levels a number of Xyz Monsters are also staples - notably Wind-Up Zenmaines, Number 25: Leviathan Dragon, Leviair the Sea Serpent, Number 39: Utopia, Gachi Gachi Gantetsu, and Steelswarm Roach.
Re: Deck Types
Yes. Also streamlining one's build, finding interesting pieces of tech, and crafting a deck that is able to meet almost every challenge with a mode of defense / counterattack. Try playing against the top decks of the format to see how your deck matches up against other decks, and make mental notes of glaring weaknesses to certain deck types.
Re: Things to Avoid
Don't make your deck more than 40 cards, please! It just makes it less consistent and all around worse than if you had only 40 cards. No, it doesn't matter if you want to include another copy of your favourite boss monster or whatever - cut something else if you have to. Single most prevalent and easy-to-fix mistake that is made in playing YuGiOh.
Re: Balance of Cards in One's Deck
I would actually say that everything is situational and as long as you can consistently find a way to win, it doesn't matter that much if the ratios of monsters to spells to traps are a little off (i.e. if you don't kill yourself because you had no monsters for a couple turns and this happens often, edit your deck, but if you run a more monster-intensive or turbo deck then it is acceptable to have varying monster counts - my decks range from 15 monsters to 30 monsters depending on purpose).
Re: Not Doing Anything on Someone's First Turn
This doesn't telegraph that they have Gorz or Tragoedia. : p It /might/ but it doesn't usually. (UNLESS YOUR OPPONENT IS BAD, OF COURSE - IN WHICH CASE THAT IS A DIFFERENT STORY.)
Re: Things One Should Know
Try to understand your opponent's deck; what combos does it have? What cards does it need to win? What can your opponent do to you that you need to watch out for? This knowledge is accumulated by exposure to the game, and so don't worry if you don't get it at first, but it is something that should be kept in mind.
Do not play too aggressively, but do not play too passively either. You need to pressure your opponent if you are letting him simply accumulate advantage every turn by not being forced to expend his resources, and every attack counts in this format (where life points matter again)! Attack with cheap, expendable monsters whenever you can to push for damage. It allows you to move that much closer to victory.
Re: Being Able to Afford Good Decks
YuGiOh is an expensive hobby. I recommend you QQ about Konami's policies of only wanting money (overpriced broken cards and super high rarities, aww yeah) and then play card games on DuelNet. (In all seriousness though the game is actually really expensive; though it's not as bad as POT OF DUALITY SOLEMN WARNING EVERYWHERE YEAH important cards for builds are still expensive.)
Dark World isn't cheap; Tour Guide is staple for them. : (
I would actually say Plant Decks are not that expensive compared to other decks like Rabbit Lagia and Dark Worlds.
OTK / FTK decks ... depends on the build.
Basically the idea here is: find good cards, buy at low prices if applicable to build. Rinse and repeat.
KOWM. (Konami Only Wants Money.) ;_; It is not recommend you buy actually expensive cards as they might / will fall out of fashion after the banlist hits; perennially good and cheap cards are better investments. (Like Gladiator Beasts; they're not likely getting nerfed any time soon).