Tire question (Newbie)

[JGunn]

Just got my first H2 05 SUT, the rig has brand new tires with the chrome hummer wheels. I want to put custom wheels on now to take advantage of the new tires, then go to 20 and a bigger tire. Question is what offset is correct, the wheel I like has -12 will that push the wheel out to far? Also if I put 20s on now, with 325 tire, with no lift will I have any rubbing issues? Thanks in advance.