You're not the first to be a little daunted by the guild, I'm sure you won't be the last.
Most RP's start out in the interest check phase on the guild, GM's will put their RP idea up and see if there's anyone around wanting to run with it and make it worth actually making into a full RP. for the most part these are your best bet if you want to get into a role play on the ground floor.
However plenty of RP's still accept new players after they've started and for that you'd need to check the OOC threads. even if the thread doesn't say if they are still accepting or not it never hurts to ask if your interested.
Finally the sections;
-Free is your basic no holds bared area, standards are pretty light and most people are just there to have some fun with some fast paced RP's (this is of course a generalisation)
-Casual is what it sounds like, there is a slightly higher expectation of standards that the Free section but its still more or less open to everyone.
-Advanced on the other hand has minimum standards, last I checked it was at least two paragraphs with a good reading comprehension level. Many advanced RP's go over these standards but you're not likely to get yelled at by anyone so long as the minimums are kept up.
Chances are one of the Guild 'Comrades' will be along before long to give you some extra assistance if you need it, they tend to do that