Just be yourself like most of us do. I did masquerade as a Skarmory back when I first joined (and I still do, to some extent), but not all of us pose as a specific character of our creation. That character usually becomes our first agent, as was the case with Agent!Desdendelle, Rina Dives, and my own agent, Falchion.

As for where you can upload missions, almost all of us use Google Docs (usually via writing the mission, getting it beta-read, sharing it publicly, and posting a link to the doc on the Board). Other sites tend to suffer from server problems, and quite a few oldbies who've been here longer than I have can probably attest that FanFiction.net is total crap. XD

It's a bit soon for that.

You need Permission for that, and getting Permission requires you to spend some time being part of the community. The Permission Givers don't give it to strangers which, unfortunately, you are at this point.