If I were you. I would first sit down and figure out which fish/corals you'd like to have. What you decide you want to keep is going to guide you through the process. If you want corals, things are going to get a little bit more complicated. I would suggest doing a fowlr
set-up to begin with unless you're feeling up to it.
Once you know which fish you're going to have, you need to plan your set up.
1. where is the tank going to sit in your house? Is the floor going to support it?
2. get lighting. If you ever plan on getting corals....just get the MH
's to begin with so you don't waste your money on PC
4. sump or no sump
5. 1-2 lbs of LR
per gallon. Where are you going to get it and are you going to use a mix of base rock and LR
6. skimmer, a Remora would work nice for a 55g
7. salt mixes. Just get the 160g mix that comes in the 5 gallon buckets. It's cheaper that way.
8. clean up crew.
11. fuge or no fuge
12. nitrate export (cheato)
13. which return pump if you use a sump.
The list goes on.