ok basics time then...im guessing that u are in the uk? in which case you need to make sure that any bottle you buy is hse tested not dot tested or it is illegal to fill it
next, as you have already bought a dive tank then you need to find out what pressure the dive tank is, will be 3000psi (207 bar) or 4500psi (300 bar) you need to make sure that the bottle you buy for your gun is able to take the fill pressure from your diver ie dont fill 3k bottles from a 4.5k tank.
then you need to get an air system...theres hundreds of different types and bottle sizes, ask people opinions of their own, get a bottle that suits the possition u play, for eg. im a front player and i only usually carry 3 pots onto a field so theres no point in me carryng enough air to shoot 10!
and dont forget you are gonna need a fill rig to get the air from your diver to your marker.
if you get stuck, go along to a pb shop and talk to somone there, they will be more than happy to help im sure