First and most important . Welcome to r2r
Being a 20 gal system did you have a idea of what you wanted in terms of fish or corals as a end goal ?
As for equipment and also depends on what you plan on adding .
live rocks or dry rocks are needed but the choice is yours which one you decide to go with .
next I would purchase 2 wave makers , heater , quality test kits ( salifert is great and affordable )
I would strongly advise against buying costly premixed water and mix your
Salt mix and a rodi system
Once you add the water , rocks and if you want sand bottom . Add a source of ammonia to get the nitrogen cycle Started which can take a up to a few weeks .
Research different methods to find one best suited for you .
last bit of advise for now .
take your time . Slow and steady wins the race , nothing good happens fast . And enjoy as it can be a very rewarding hobby
feel free to ask questions and to post pictures along your journey .