Well you would need salt mix, hydromoter, salt test kits to start with.
Depending on what you have in the bottom of your tank if its gravel then you will probably be better changing that out to crushed coral or sand. I would suggest 4-45lbs of live rock aswell for not only filtration but also look. You should pickup a decent skimmer to add to the filtration.
If you plan on wanting corals then you will need to upgrade your lighting because no doubt you have a single florecent tube. A double florecent fixture would be ok for just the live rock but corals need more potent light.
Is it hard to keep? Not really. But there is a level of devotion that you will need. Regular water changes, daily to bidaily topoffs of evapration for example. The fish are much more expensive but the trade off is you keep less fish in the tank.
What fish are drawing you toward salt?
If your serious then there is no replacment for a good saltwater book. Check out our saltwater library linked above for some suggested books. I like the book entitled "The Conscientious Marine Aquarist" by Robert M. Fenner. This is a great book for the person getting into the hobby.
