If you are asking the question for your own personal use, a set of allen wrenches. Everything bike related is metric. Get a cassette tool that fits your cassette, probably one big adjustable wrench. Go around your bike and see what parts it uses and get tools to work on those parts.
If you are looking to work on other people's bikes, fuhgeddaboudit. If you don't know what tools you need you aren't up to the task. I looked at a neighbor's bike last week and my hands got so dirty I swore I'd never touch another bike other than my own.