The Park (Feedback Sports also make a nice version) 3 way hex wrench (Allen is a brand) are a very good tool to begin with, most basic adjustment on a bike can be done with one of these, and a flat blade screwdriver and a JIS screwdriver (for Shimano RD's).

After that, would look at the tools your bike needs, avoid buying a 'bike' tool kit, as these are often only useful for older bikes with headsets and non-external type BB's.

No need to buy bike-specific tools for the basics, as there are plenty of good industrial tools around which are cheaper and do exactly the same job.