Yes they sell full caps and people do wear them. But keep in mind that with full caps, it is slightly more work and a little harder because you have to attach the thing all around your head. The nape area of the neck is a problem spot for full caps. Some people leave some of their own fringe hair in that area to avoid the attachment of the nape problem. It might not be a huge problem, just a consideration and something guys with full caps talk about.

you learn by trying. You are young so you wont look obvious even when you make mistakes with density etc.
There is nothing really to it, You do the same things with a full cap you would with a partial when ordering. template, hair sample, density, hair direction etc.