Tamiya's 1/35 range is hard to beat for a first build. The Tiger I, Sherman and Panther are all great kits with clear instructions. Avoid kits with individual track links for your first go — go with rubber band tracks or click-together.
Do I need to paint it?
You don't have to, but an unpainted tank looks pretty ordinary. Even a single base coat and a wash makes a massive difference. Tamiya spray cans are the easiest way in — no airbrush needed.