Wizzy, DBNB, and Yummy already gave the best advice. I'll be a little more specific, since sometimes it's hard to jump right in from the top. Generally speaking my advice would be this: Read first, then write, then hit the forums.
Reading first is pretty self-explanatory; becoming well-versed in the games around the site gives you a good feel for what the community is like and helps you figure out where you'll fit in. Writing next is optional; I wrote lots of reviews and my first two storygames before really becoming active on the forums, and I felt like that helped a lot with navigating them for the first time. The forums are best saved until you're sure you know your way around/have lurked for a bit, since like any community, there are a lot of obscure trends and community rules that a newcomer won't know about.
Reading advice:
Start with the featured games in your prefered genre, and then if you like a game, check out other games by the same author.
I strongly suggest leaving reviews on every game you play, even if it's just a couple sentences, particularly if you're planning to be active around the site. Authors appreciate reviews and helps other people get to know you. If you have the time and motivation to leave thorough reviews authors will really appreciate it; good reviewers are as appreciated and often more appreciated than good authors. Authors most appreciate specific and useful feedback; don't just say "good", say what specifically you thought was good and why.
Here's what I would call the "site classics", though as Wizzy, DBNB, and Yummy say, you know your own interests best.
Eternal is an epic-scope dark fantasy game, and holds the well-deserved title for most popular game on the site. One of my favorite stories, period. (That said, it's very long, and I don't recommend it if you're twelve.)
Dungeon stompage and Dead Man Walking are both by the same author; both are more game-like storygame with stats and a wide variety of playthroughs.
Writing advice: (if that's your thing)
- Read first. You'll have a much better idea of how to format a storygame after seeing what others have already done.
- Start small. Don't sit down to write a 200k fantasy epic. For your first game, aim for something like 20k words or less.
- Write what intrests you, not what you think will be popular. For one thing, you're probably wrong about what will be popular, for another, writing something you're not interested in is a surefire way to run out of motivation.
- Have thick skin. Vivisecting authors in the comment section of their games is a favorite CYS sport, so be prepared to be bluntly told everything wrong with your game. The upshot of this is no one will give you undeserved praise, if someone says they like your game, they really mean it.
Best of luck! Hope you enjoy