Do lots of research. I'm about to make the jump but only after 3-4 weeks of research (at least that much).
There are so many different types of systems, pumps, radiators, etc. Each of the parts should match the type of system (high flow/high pressure, low flow/high pressure, etc.). The barb size will have an effect on the system and they need to match with tube size.
Also, do you want an AC (plug in the wall) or DC (plug into PSU) pump? How big is your loop (CPU only, CPU+GPU, CPU+GPU+NB, etc.)? These things will affect the pump because more items in the loop require more pressure. You need to make sure you get a pump that can handle your setup.