Thats a very controversial topic, but here is my advice.
When just starting out, you want to know when your cycle is complete and your biological filtration is ready to handle the load of tank inhabitants.
At this point, monitoring Ammonia Nitrite and Nitrate is essential. Most hobby grade kits are ok for this but I like API's.
Once you get established, it becomes a little more complicated depending on wha type of tank you are going to have. Fish Only, Fish with live rock, mixed reef or a full blown reef.
Then we can tell you what test kits are recommended for measuring the big three. Calcium, Alkalinity and Magnesium.
Lets not get ahead of ourselves. Get your tank to cycle first, then we can go from there.
Good luck
