Loads of things! First you need to decide what type of filtration you wish to use and there are many ways to run it. I think using a sump is a good idea because it adds water volume to the system and is a good place for hiding all your equipment out of the display. You can of course use a hob filter or just a skimmer, up to you. There will be many people to help you which way you decide. I would recommend a protein skimmer on that sized tank, you will also need a couple of powerheads for flow, a heater and lighting. For a reef be prepared to drop some serious cash on lighting. You will also need to look at test kits and a refractometer for measuring salinity. Investing in a rodi system for making water I think is essential for a reef tank (no tap water here) of course then you are looking at sand (if you like) and rocks, try and aim for 1 to 2 pounds per gallon. Most importantly do plenty of research!