Alright. I'll start with joining an RP, since that's a pre-requisite to posting.
Joining an RP is easy once you've done it once or twice. The first thing you're going to want to do is find an RP you want to join. There are quite a few ways to go about this. Go to the Roleplay Corner and look for the sticky thread "Check to See if Your Roleplay has been Accepted (v.2)". This is more or less an accurate way to find new RPs, but hey! You might not want a specifically new RP, as their lifespan tends to be unstable.
There are two other ways to find an RP. If you go to the Role Chit-Chat Thread, also a stickied thread in the roleplay corner, you can sometimes find GMs advertising their RPs, which is arguably the easy approach to finding an RP.
But if you really want to take things into your own hands, the best way to find an RP is just to browse the Roleplay Lounge! The roleplay lounge is a sub-forum inside the Roleplay Corner and is where all the OOC for RPs go. It is also usually where you are supposed to sign up for an RP and where an RP gives you most of the information about itself.
The first thing you're going to want to do is read up on the RP and think about how your characters will fit into it. You may need to edit your characters to fit into any given RP, though creating a new character avoids this issue. The GM will love you if you give their rules and plot and stuff a good read over before signing-up.
You want to create a character that meshes well with other characters, which requires a bit of forethought in the character creation stage. You want a character that you will both have fun playing and a character that you think will come off as interesting to play with for others. Feel free to ask any questions you mayhave before submitting something
Finally once you've filled out the Sign-Up, or SU as it's often called you can post it in the OOC thread. The GM will look it over and give you their thoughts. They may ask you to change some things. At best they just want to know more about your character and just want you to elaborate on what you already have. You can practically think of that as a compliment. At worst however you may have missed a rule or plot point while reading over the OOC.
Don't worry too much about impressing everyone with your SU. The Roleplay Corner is a pretty friendly place, so even if you do make a bunch of mistakes it doesn't mean much for your reputation, especially since you're a new face. Give it your best shot but don't stress too much over it. If you can impress even a couple of players you've done pretty good.
That's an in-depth rundown on the process of joining an RP. Are there any questions I can answer for you before I move on to posting?