Play by Post, as it's called (or PbP) is easy, but requires some patience. Forum games are eeeeeever so much slower than tabletop, so if waiting drives you mad it may not be for you. If you don't mind, read on.
Step 1: Find a group! This forum, and some others, have places for this. Play By Post
is where I currently run one, and play in two games. Step 1 could also be "make a game" but since you seem lost I presume we're not trying to do that.
Step 2: Figure out if the group is looking for players and what rules/houserules they're using. Pretty much like you'd do in real life.
Step 3: Pet a cat! Cat's are cute! VERY IMPORTANT STEP!
Step 4: Make a character sheet. People usually have various templates floating around. Steal their design!
Step 5: ????
Step: 6: Profit! I mean play!
The trouble is the slowness, since you need to wait for people to post which can take on the order of days. At the same time don't slack and forget about it; that kills games and you don't want people to delay with you after all.
It's not a rule and it varies with opinion and game, but having a nice writing style is a plus. If you're in a game and it's full of rich fancy writing or exciting action, it's so disappointing to see a post like "I move and hit with sword, 5." Since this is a slow and written environment you can afford to be descriptive with what you do and say, using different fonts, or other tricks and nice things. That one's just my opinion.