Welcome to AA!
Here's a great link to get you started - Stock list and tips for maintaining your SW tank.
It can be pretty confusing starting out but if you do your research you'll see it isn't as complicated, depending on what you're looking to do of course. The equipment and set up will be based on the type of tank you want - reef or FOWLR
(fish-only with live rock). You can keep your powerhead and probably your filter as well if it is relatively clean. I recommend buying new aragonite sand to use for the substrate and a lot of live rock, with helps with your filtration and is a vital part of the aquarium. Lighting will depend on whether you want to keep corals or fish only. Maintenance will vary again, but consists mostly of cleaning and regular water changes, just as with freshwater aquariums. You can look up what fish you like and ask about them here. Check out LiveAquaria.com, they have a good selection of fish to browse and a compatibility chart as well.
Go through the link above, it will cover a lot of your questions. I assume you know about fishless cycling but if not, check out the Articles section of this site for a good writeup. Saltwater aquariums can be very expensive and require a lot of patience but are really rewarding once set up. I wish you good luck with your tank!