This question actually gets asked a lot. So I’d suggest doing a search on this site.
As for finding a good school, well, it really depends on what you’re purpose for training is. Are you looking to just get in shape? Do forms competitions (weapons of empty hand)? Compete in a combat sport (boxing, muy thai, grappling, MMA)? Or, self defense?
If you purpose is either of the last two, then I’d suggest finding a system that teaches a well rounded curriculum. Meaning that they teach all the ranges of combat (everything from psychological to ground), teach all the arsenals of combat (striking, grappling, biting, nerve attacks, eye attacks, body handles, and environmental weapons), teach how to avoid fights in the first place (cerebral, verbal, and postural self defense), and lastly that the instructor not only can show documented proof that he/she has trained under other martial artists of high skill, but can also teach what he/she has learned.
As for actual systems, I’m actually not going to go there, since people tend to take the topic of systems personally. If you can find a place that fits all of the above criteria, train there, regardless of what the name on the window says.