I found a game or two that I would be interested in joining, but I'm a bit unsure as to the proper etiquette for joining one. I see the "Request to join" button, which takes me to a message screen much like this one.

What should I do next? Would a simple "hey, this game sounds fun! Can I join?" suffice? Or, do I need to view it as a "cover letter," where I lay out my experience and prior knowledge and a writing sample and etc, etc? Do I need to "sell" myself as a player or something?

In case it's not obvious, I have practically no experience with PbP and want to make sure I don't approach things in the wrong fashion.

most games will have a thread in them, visible even if you aren't in the game yet, that will tell you what the GM wants in an rtj.  Usually it's something like a name, character concept, and other specifics depending on the game.

if there isn't anything like that at all, and the game is requesting players, a good start might be to say hello, and ask what the gm wants from applicants.

What he said...

Don't forget to check the Game Info link, too.  That may also have "news you can use".

Hey, as long as you are polite, there's no real wrong way to ask to join a game.

And the fact that you think to ask this question puts you leagues ahead of the average noob. :)

Yeah, what he said.  Be real, be polite and civil, and that'll put you miles ahead.

Here's hoping you find the game that will make you want to play!

All great advice! I'd add that if a GM has included a particular format for their RTJ, use it. This might seem like a no-brainer, but for popular systems like Pathfinder or D&D, you might get dozens of RTJs, and having them all lined up the same way can take away quite a bit of headache.

It also shows you're willing to put the time into a handcrafted RTJ, rather than copy-pasting the same one you sent to fifteen other games. Shows the GM you care.

Most important etiquette is just reading whatever public info the GM put up before applying. This will help you to figure out if it looks like the game for you, and is the best way to find out if the GM does want any specific info included in your request.