Well, the answer would pretty much depend on where you're starting from. Much of a bike can be assembled with what I'd consider generic tools, although they are available as bike tools too. You'd need some Allen keys, maybe even torx. Screwdrivers - phillips. Some specials, like bb and lockring tools, cone wrenches. Bike specific cable cutters will make the job easier, although a Dremel will also do the trick. Reamers and facers for headsets and bbs you have to be fairly dedicated to buy. Even I bring those jobs to the mechanics.