A lot of things.
fill your tank with R/O DI water. after you put in your sand and aquascape your rock.
add salt
check salinity
add Dr. Tim's one and only.
at the same time, add some fish
I KNOW I WANT TO ADD DAMSEL FISH! LOL OMGZ
no, don't every cycle your tank with damsels. they are terrible fish
add some blue green chromis, you'll actually be able to pull these guys out of the tank once it's cycled.
cycle the tank
now you can start adding real fish.
when you understand how to dose kalkwasser or have a properly functioning calcium reactor (they are basically the best thing in the world) you can think about growing corals. if you do water changes you can grow corals in smaller tanks, but you need to be adding the right nutrients.
go to reefcentral and spend a couple days reading up on what they have to say. there are a ton of different schools of thought about basically everything, and they all have worked for someone somewhere.
i would recommend buying a controller for your tank. You have no idea how much money this can save for a reefer. but a controller before you fill your tank.
there is just too much to write here.