I spent the 1st year out of college working at a bike shop–it was about the only thing I was qualified for. My experience b4 I got hired was limited to building & maintaining my own BMX & road bikes. I lived in a small town growing up that had no bike shops–I learned by having to do it. No internet, and before the Zen bike book came out.
I was fortunate that I had a name for myself in the midwest due to triathlons, and I knew plenty of people. Convincing a shop that catered to the college crowd at IU (they sold truckloads of <$600 Mt. Bikes every fall) to give me a shot as a wrench wasn’t too difficult. The first few months, all I did was build bikes. That gets old really fast, but when you are paid $10.00 per bike, you learn how to do everything quickly. Oh, and you will change lots of flat tires (those were sweet–made $1.95 per)–somedays you would have 20-30 different flats come thru the shop.
After the fall rush, and the free tuneups on the bikes you’ve built (which you don’t make squat on, so you make sure they go out the door perfectly with cables already stretched), you get a shot at doing whatever the regular wrenchs don’t want to do (can you say hub & BB rebuilds) or whatever takes the least time & pays the least.
Its not rocket science, but you do learn to do things correctly. Having the proper tools at your disposal is awesome.
If you can build a road and a Mt. bike frame up from scratch, you can wrench at a bike shop. Some of the specialized jobs, like rebuilding mt bike forks, take some time to learn (and that technology changes all the time + every manufacturer has a different set of specs), and if possible I would focus on a shop that does primarily road/tri business, as you are basically dealing with two manufacturers for 90% of the parts & there is uniformity in how everything operates).
There are some things that to this day I don’t know how to do such as how a 3 or 5 speed hub works, but 99% of the day-to-day is replacing cables, adjusting brakes, replacing headsets, rebuilding hubs/bb. It certainly isn’t rocket science, and as long as you have a basic understanding of how things work (if you built erector sets as a kid you are set), have good attention to detail, are not afraid to ask if you aren’t sure, and don’t mind shitty pay & getting grease everywhere, its a pretty laid back job.