Check out the articles link on the home page.
Are you looking to do a nano or a regular SW
setup? If nano, I would skip the 20g tank and look into a 12g or 24g nanocube. For the money it will probably be a wash for what you will put into the 20g with proper lighting and filtration.
Otherwise the only initial advice I would give is to look into setting up as large a sw
tank as you have space for/can afford. Unlike fw
the larger the setup the easier it is to maintain due to the fact that you have more water volume and it is a little more forgiving.
Otherwise equipment will include:
power head for more water movement
Lighting 3-5 wpg
As for inhabitants it really depends what kind of set up you want. If you go with the 20g you will be limited to 2-3 smaller fish.
Here is a good link to check out some basics on fish and corals for size and lighting requirements.
Otherwise hang around here and do some reading. Research is the best place to start.