Best piece of advice I can give you is patience. You MUST have it when you move to saltwater marine and especially reef tanks. Nothing GOOD happens quickly with this particular hobby, only the BAD.
A few pointers:
-Protein Skimmers are very nice, not required but highly recommended
-You WILL have an ugly brown diatom bloom after the first few weeks, expect algae for the first 6 months
-Don't use a BioWheel or any other Bio media, they end up being
nitrate factories
-Nitrates are not tolerated in reef tanks, even the smallest amount can adversely affect your corals
-Salt does not evaporate, so use freshwater
RO only for your top-off water
-Most Tap Water contains small amounts of phosphates and other metals which aren't good for your marine tank, use RO water from the
LFS or local grocery when mixing your saltwater
-If you do a reef tank, you need to track a whole new slew of parameters (
calcium, PO4,
alkalinity,
magnesium etc)
-No, play sand isn't a good choice for your sandbed, most of it contains silica which combined with other elements in saltwater setups will turn your sandbed into concrete eventually. You want something that will
buffer the
pH, like a good argonite based sand.
-Don't use freshwater substrate
-cheap doesn't work in this hobby, you'll suffer from it. I'm cheap, I've suffered trying to save a buck

Prepare to spend money.
Well, I know there is a lot more to it and that's an understatement! Read read read and read some more when it comes to saltwater/marine setups. A great book is '
the new marine aquarium' by Michael S. Paletta. It's rather like the saltwater bible IMHO (no religious slur intended) You can find it at your LFS.
I've found that while I still love my freshwater setup and it's doing great BTW, there is nothing more intriguing and mysterious than a Marine Reef system, there are SO many different species and SO many things to research and just watch for hours and hours.
Hope this helps!