Hello, and welcome to PlanetSide Universe!
Outfits in PlanetSide 2 are basically the same thing as a clan or a guild in other games; a group of people playing together. Different outfits have different goals and sizes.
To join an outfit, you need to get an invite to the outfit.
Different outfits handle this in different ways. Some run platoons in which they recruit, others recruit here on PSU or on the SOE forums.
You yourself choose the role you want to play in PS2.
You can be a pilot, a sniper, a tanker, medic, engineer... You get the point, you can do lots of different things. Try different things and see what suits you the best!
EDIT: Is this your character?
https://players.planetside2.com/#!/5428089603121866817/
Seems like you already found an outfit.
Anyway, if you have more questions, or if you need more explanation of something, feel free to ask!
Micro