I've slowly collected tools over the last almost 30 years. When I was a new rider I wish I had got a basic tool kit like this one right off the bat:

Nashbar Essential Tool Kit

With that kit you can install chains, remove cassettes, fix flats, remove cranks, adjust headsets, have an allen tool for every bolt on your bike, ect... It blows when you need to work on something and you don't have the tool. Then you make an emergency run to the bike shop and buy a tool for triple the price.