The first thing you should get is a good bike repair manual like Lennard Zinn's "Zinn and the Art of Road (or MTB) Bike Maintenance" or the Park Tool's "Blue Book" and read them cover to cover. They will familiarize you with the range of bike repair and maintenance procedures and the required tools. Then decide what types of bikes you want to work on and what level of complexity you want to tackle. At that point you can begin to assemble the needed tools.