For a FOWLR
you do not need special lights.
As far as stocking goes. Use this as a guide. Most will live in a 10 gallon, but I believe yours is close to 20 gallons.
For a filter I would use an HOB
Aqua Clear filter that can do 50 to 70 gallons of filtration. Heater of course! One nano power head, and 20 pounds of live rock with at least 30 pounds of sand for a good sand bottom!
As far as a skimmer goes, they are good to have! So just get a skimmer that is small HOB
. A reef octopus skimmer will work well! I think there is a smaller one that will hang on the back you can buy.