Specifically, this is the FAQ on Permission. Generally speaking--as it says there, and as Boarders tend to repeatedly say--we like it if newbies hang around for about a month*, and are active on the Board during that time--joining discussions, maybe starting a thread or two, and talking about things other than Permission--before actually submitting a request. We like to know who you are; we like to be able to see your username and think, oh, hey, that's the one who's into the Hunger Games but hates Peeta's storyline and had that awesome theory on how Sherlock survived his fall! Also, speaking from experience, it helps to start coming up with your agents...and then spend a lot of time developing them before you actually present them for approval. (By developing, I mean brainstorming, thinking about how they'd react in various situations, even writing some backstory for them--anything but writing a real mission, honestly, as that's why you need Permission in the first place). Trust me--you and your agents will both benefit from the time you spend together.
But yeah--basically, hang around for a bit, chat, get to know us, develop your potential agents--feel free to ask questions about anything you're unsure of, we're happy to advise--and then, when you feel ready, put together your Permission request (writing sample, agent bios, and a badfic or two that you'd like to take down), preferably show it to a beta or two, and then post it.
Aaand, now that I've really made myself feel like an oldbie and a fixture--take a look at the wiki article, because it's a, more detailed, and b, probably a little easier to understand/more comprehensive.
Best of luck! And, um, welcome...not sure I ever welcomed you. Please accept these Gummyblee Pens and Spaceships as a Welcome Gift...
~DF
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*One month is basically there as a kind of standard waiting period--in reality, it varies a little by person: some may ask after three weeks, some after a year--but it's there so that people don't come, hang out for a day or so, and then ask for Permission. As I said--or will say, depending on when you decided to read this footnote--we like to actually get to know you a little before dealing with a Permission request.