IMO, the best test kits to start with have Ph, Nitrites, Ammonia, Nitrates tests for saltwater. API is pretty affordable. You need a thermometer and something to test the salinity. A refractometer is a good investment, but I think most people start with a hydrometer.
These basic levels can be enough for about 6 months while your tank is stabilizing. Once you're ready to add corals, you'll need more test kits.
Have fun!
Denise