Welcome to Canreef.
I think the best advice I can give is, look at lots of setups and see what sort of things you like, I find I get the best ideas just by looking at what others are doing.
A 120g is a very decent sized tank. I'm jealous, I wish I would have jumped straight into a tank like that when I was starting up. I started smaller and worked my way up to larger and I'm still working my way up (biggest tank is a mere 90g). One day, maybe something larger, who knows.
Anyhow best of luck with your setup. I know you'll get lots of great advice along the way. But if you start to get overwhelmed, then don't worry, there's a lot of info to digest. Like I said, look at lots, invite yourself over to places, and see what ideas work for you.
cheers