You need a tank (10-20 gallon, or custom acryllic if you want). A return pump, and an overflow for your main tank.
Why are you thinking of adding a sump? Fuge? Equipment repository? Only extra bio filtration? The design of it depends a lot on that.
55g reef, 2x250W 10k MH
+ 2x96W PC
actninc, 10g sump, AquaC Urchin skimmer, 65lbs LR
xenia, mushrooms, wels. open brain, fungia radiata (orange), green eyed zoos
flame angel, mated pair of false perc, 6-line, firefish
20gH electric yellow cichlid