Metric combo and Allen wrench sets are a good start: combo wrenches from 8mm to 17 or 19mm, 4, 5, 6mm Allen wrenches and perhaps 7 or 8mm depending on your bike. Bike-specific tools depend on your specific bike and what components it has. Tools for servicing freewheel or cassette, hubs, crank, and headset will depend on what you have.