Glad to see you're taking the time to read how to play. Although playing a text based RPG like this is usually quite simple, rules do vary from forum to forum.
The concept is quite simple. This is basically colaborative fiction. What you do is make a character (who you will focus on during your posts), and then you write about his/her adventures. However this isn't a personal fiction forum, this is collaborative. Which means you have to take into account the actions of other players.
Here's an example of what an RPG in action sort of looks like;
|Bob then accidently spilled the glass of water.|
|Seing Bob spill water on the table made Jill laugh.|
|Bob saw Jill laugh at him, this embarresed Bob.|
"I think I'll clean this up" Said Bob as he grabbed a napkin.
|"That would be a good idea" Jill said to Bob. Jill being a kind person got Bob another napkin.|
Hopefully that get's the idea across. If you're still not entirely sure how one of these text based RPG's works, I recommend that you go and simply read some of the missions in progress and learn from what you read.