first, i would recommend some search terms for you:
beer gun, beergun, CPBF, counter pressure bottle filler, force carbonation
with that said, there are many ways to bottle your brew from kegs. most expensive would be a CPBF, cheapest would be put a piece of tubing on your faucet or picnic faucet, and pour directly into a bottle then cap.
you can either prime a keg with sugar like you would with bottles, or you can force carbonate. i prefer to force carbonate as i am lazy and impatient. some like to conserve co2. i have 6 co2 tanks, one of which is a 50# tank, so i don't really worry about conserving co2, and it is really cheap.