a good bang for the buck general test kit that will have all the things you will initially need to test for would be the API Saltwater Master Test kit. I was going to recommend the Tetra basic test kit but it has some things you dont need like Low range PH and General hardness which are freshwater test kits and it lacks Nitrate. As said before once you get everything up and running and you want to start keeping corals then you want to start testing for things like Calcium, Magnesium, and Phosphate. If you plan on keeping corals in the long run then you can buy everything at once in nifty carrying case by getting the Sera Marin Test Kit which has tests for pH, KH (alkalinity), Ammonia, Nitrate, Nitrite, Calcium, Phosphate, Copper and Iron. The only thing that kit is missing is Magnesium which you can buy separate. Copper and Iron arent usually tested much in saltwater but its one of those things you can test every great once in a while or if things start coming up weird and you want to eliminate suspects.
