Hey there Purple,
What I would strongly recommend before doing anything else is getting the following test kits:
pH
ammonia
nitrite
nitrate
And if you want to get serious about keeping corals rather than just inverts, you also need:
calcium
alkalinity
This covers most of what you will likely need. Now the next step is learning how they all interact. For example, calcium, pH, and alkalinity are all interrelated to a certain degree. I strongly urge buying a book that explains them more, like say
Goldstein, R.J., Marine Reef Aquarium Handbook, Barron's Educational Series, Hauppauge, NY, 198 p., 1997.
It is a good book that was only about $15 from BorderBooks (you could probably also get if from amazon.com).
Welcome to the board,
sam