I'll give you the same piece of advice I gave a friend of mine when he was considering setting up a marine tank-- get nutrient testing kits before anything else & test your intended water source, typically your tap. If there are any semi-significant levels of Nitrates, Phosphates, or Silicates in your tap, you would do well to purchase an RO/DI (reverse osmosis de-ionizer) water filter & head off numerous algae battles before they start.
After that, make a decision-- do you intend to keep
any kind of coral, anemone, or other 'sessile' invertebrate in the future? (yes, yes, I know anemones aren't really sessile

) This should influence the kinds of fish you add to your tank, of which you should research thoroughly
before you add them. Go back to your LFS, look at the fish you might like to see in your tank, then go read up on them & see what their compatibilities are.
Saltwater Aquariums for Dummies was an integral purchase in my learning path, as well as the Concientious Marine Aquarist suggested above.
This should give you plenty to chew on for the immediate future
