I may be able to help you out, though my weekends are stacking up to be pretty busy for awhile.
Water is really the only thing that needs to go in it, unless you want to create a sump/refugium, in which you can put some macro algae in there, and even sand if you want.
Also its a good place to be able to put your heater, and skimmer if you have one and it will fit (I'm thinking on a 10 gallon probably not) A lot of people will put baffles in there too, which I did have in my old 10gallon sump, but I don't now in my 29. They are really not required.
The goal is to get water from the tank down into the sump, and then water from the sump back to the tank. The overflow is what helps you get it down to the sump, and the pump is what helps it return (thus called a return pump)
Basically as a bare minimum all you are doing is adding some water volume to the overall system, and with such a small tank, that's really the only effect you are getting out of it. Don't get me wrong though, it is still a huge improvement.