Welcome to Librarium Online!
I am new to Warhammer 40k. I actually started playing 12 years ago, but after only playing twice, the models were put away and never again taken out until a few weeks ago. So happy to be back into the hobby.
Sounds like the typical Warhammer 40k story haha. It is an on and off hobby. Too bad life doesn't give us more time to game/paint. A lot has changed since you've been around. I'd suggest first getting a rulebook and codex to refresh yourself on the rules and changes that happened over the last 12 years. There is a lot to catch up on before trying to decide an army list.