[welcome]
Im helping a friend set up a 20l reef tank, heres what we have so far....
Lighting - 130w PC, we are keeping corals so more lighting is needed for ours.
Skimmer - Seaclone 100
Tank - 20L AGA
Substrate - 20lbs finely crushed coral
Powerhead - MJ 400 for flow
Salt - 50g mix for now
Hydrometer - cheap plastic floater (better than nothing)
Test Kits - for FOWLR get (at least) Ammonia, nitrAte, nitrIte, and pH
Heater - sutable for size of tank
Thermonoter - got the digi coralife $5 at Fosters and smith
Live rock - 20lbs
Stand - she had one
thats the basics, here are more things you need or make life easier...
Turkey baster (the best toy you can buy for a fish tank

), tweezers, Timers (for lights), Powerstrip, Fish food, Magfloat, buckets + hoses (Water changes), and last but not least PATIENTS
good luck