first time with saltwater

To clean the 20 long, you would just have to dump everything out and just wash it out with water. Don't use any cleaning products to help get it clean, other than like a scraper or something in case something is stuck on the glass.

For the sand that should be ok, unless you want a DSB where you will need more. Add enough of the sand to the point where it is about 1-2 inches high. Try to keep it as low as possible.

The salt should last you for some time. I use the same for my 36 and its only halfway through.

Are you getting liverock? It is optional but a major plus and one of the basic things to add to your tank. It serves as the main bio filter, look of the tank, your livestock lives on it, and it is full of life. It is seriously a good investment. I would recommend the rock over the canister.

You will only need good lighting if you are going to go reef and add corals. If you go fish only you only need light to see, and FOWLR needs light if you want help coraline grow, and keep some inverts.

An RO/DI unit is necessary so you can make your water changes, unless you can buy or get RO water from a store or someplace. I just use a Tap water filter, it does the same, except you need to change the cartridges ever 50 gallons or so. Works good for small tanks, but it also depends on what is in your tap.

You need to test in the beginning for Nitrites and Ammonia, and will regularly need to check Ph, Nitrate, and Phosphate. Calcium and Mg are also important too.