What to test for?

Not sure what you are asking - if you mean what chemical tests should you make then reading the sticky above about ferts and so on should help you out. pH, NO3, Iron, CO2 etc. are things you need to know about....

NO3, PO4, pH and KH are the most important things to test for. I've heard several people suggest that Fe is a waste of money and I tend to agree. Being the cheapskate that I am, I'll let the plants tell me if they need more.

Pick up a 'master test kit'. It will have tests for gH,kH,nitrites,ammonia,pH, and probably nitrates. It is cheaper to buy them all in the same kit than individually.
If you either have many of them, or have an established tank which probably won't need to be tested for ammonia or nitrites, you want to have kH,pH,nitrates and phosphates. The kH/pH combo is important, especially if you decide to inject CO2 at some point.
N and P should be tested regularly until you learn your tank's needs and how much of each to dose.
K is hard to overdose if reasonable care and research is done, and Fe test kits are, at best, difficult to use. I don't use either and would suggest not wasting your money on them.
I use Aquarium Pharmaceuticals for N and pH and Hagen's(Nutrifin) for P. They are relatively accurate, IMO.

Len