Most bike tools are inexpensive, but still by the time you buy all the specialized tools needed for a full tuneup, you will have spent well over $100 (think cable cutter, bottom bracket tool, cassette tools, chain tool, hub and headset wrenches, spoke wrenches, etc). That assumes you already have non-specialized tools like metric allen keys, metric combination wrenches, pliers, screwdrivers, file or Dremel, etc. It also assumes you rig up a homemade work stand (using ropes/hooks to hang the bike from the ceiling, by the handlebar and saddle, works sort of okay). Bike repair is a lot less complex than, say, auto repair, but there is still a lot to learn so you'll have to buy a good book and spend many hours reading, viewing videos, getting it wrong, asking here, re-doing the job, etc. If you are careful and do your homework, most likely nothing bad will happen.