Since you are looking into a clownfish tank, I would recommend looking into an "all in one" tank like a nano cube, bio cube, ect. Something like that will give you the least upfront investment, but still set you up for success. Some come with a built in protein skimmer, but most have it as an option with a space in the back for it. Numerous manufacturers make small skimmers for these types of tanks, and while they aren't absolutely necessary, they do help your success.
Most problems people have usually come down to water quality (nuisance algae, diatoms, cyano, ect) so I would recommend not using dechlorinated tap water when mixing saltwater, and starting of with RO water from the start. If you aren't ready to invest in a dedicated ro or ro/di setup, you can purchase the water at the grocery store by the gallon as "drinking water." Some stores have a machine and you can fill your own bottles. A few five gallon bottles and you'll be set.
Go to the library or look on amazon for a book or two. The Consciences Marine Aquarist is one, just make sure it's the updated version. Most of the fish keeping practices you already know, but it is a good reference for you to see the differences in saltwater aquariums and answer some of the questions you may have.
Good luck!