What you use for substrate depends on a couple of things, starting with how much you want to spend.
One of the best substrates around is Seachem's Flourite. It is Iron rich and a complete substrate that will last for years and years. It is a compete substrate, but is expensive.
If you want a cheaper alternative you can use sand and/or regular gravel, but you don't want to go too fine with either. Sand, in finer grain has a tendency to compact and can, in the long run cause problems over time.
If you decide to use sand or gravel, many of the hobbyists use a layer of laterite on the bottom followed by your gravel of choice.
Or you can skip the laterite and go with a very(couple of handfuls spread over the bottom of the tank) thin layer of peat moss and put the gravel over top of that.
My suggestion, if it's within your means, is Flourite or similar complete substrate. You'll be glad you did in the long run.
Len