Cycling is super important for saltwater aquariums and is what makes your sand and rock into live sand & live rock. Don't forget that your live rock is your main biological filter followed by your live sand. This
thread here on MFK (in this section's stickies, actually) explains it pretty well and provides instructions.
I forgot to mention in my equipment post that the plastic egg-crate stuff that's used on office lights makes a good bottom layer for the aquarium; it helps to hold the rock in place while helping to prevent you from breaking the glass by dropping a rock (as well as other benefits).
Also, please post up what you have as far as filters, light fixtures, and other equipment so that we can make recommendations as to what you should/shouldn't use.
Macro algae, specifically chaeto, is great for helping with nitrates and phosphates plus it provides a home for copepods and other plankton. You can easily convert one of your HOB filters into a hang-on refugium by adding a clip light and some chaeto; you could also make a refugium within the sump should you choose to utilize a sump.
RO/DI water makes it a lot easier because it removes all the impurities from the water thereby preventing you from introducing any nitrates, phosphates, dissolved solids, and so on. However, for a FO or FOWLR, you can get away with dechlorinated tap water in most cases depending on your water quality; once again, you're going to have to watch the parameters more in this case, especially phosphate.
I've probably left some other things out, but that should give you a pretty good start from an equipment standpoint. Now then, on to your livestock...