Definitely start off with tire levers, a patch kit, a frame pump, and a saddle bag or something to carry them on the bike so you won't be stranded on the road by a flat tire. I got a frame pump first and then a nice floor pump as well since it is such an often used tool (to top up tires everyday). A multi-tool like the Topeak Alien II has probably every allen key you'll need. I think that's the minimum you should have so you can fix flat tires and make basic adjustments.