Originally Posted by
positron
A simple multitool will allow you to do many things on most modern bikes, including adjusting brakes and derailleurs and tweaking bike fit. As you get more comfortable with simple mechanical stuff, work your way up to more advanced skills like replacing chains, working on older bikes with loose bearings etc. Buy good quality tools as you need them, and they will last for years.
You can do a lot with a multi-tool, but I think you'd be happier with full-sized tools. I'd grab a pair of Pedro's tire levers, a multi-bit screwdriver, and a set of metric Allen wrenches. Buy a Park CT-5 chain tool if you want to work on chains; it's not full-sized but it'll get the job done. You should be able to get all of this stuff for $30-40, maybe as little as $20 if you're a talented shopper.