Theory only does so much. You can read a gazillion field manuals, but that won't make you any more experienced if you don't actually TOUCH the weapons.
If you're of age and verified, buy a gun that you like. Think of it like girls... are you an AK man? Armalite man? Brunette? Blonde?
Play around with that. Learn its idiosyncracies. If you don't like it, sell it and trade for something else. Or you might like it and decide to keep it. Buy something else to expand your collection and repertoire.
Go to a game. That is the best way to learn what is good and not good for you and your gun. You also get exposed to other models. Ask nicely to touch them. That will give you an idea of the differences in handling.
In terms of performance, that is the gearbox's job. There should be more than enough guides here for you to read about upgrading internals. Don't upgrade until you have at least played in stock form.
Contras A-06: F**k it, we'll do it live.