>>1031610>What should I learn immediately?This would be a great question to ask your coworkers and/or service manager as it will show them you're enthusiastic about learning as well as giving you a better idea of what the most common repairs are at your specific shop. But in anycase, here's my advice from my own 3-ish years of being a wrench at an LBS:
Get good at fixing flats quickly and efficiently as the vast majority of walk-ins will most likely be flat tires. Learn to check tires for damage while you're doing this so you don't hand a bike back to the customer with whatever caused the flat still stuck in the tread.
After that, I'd get familiar with adjusting brakes and replacing pads (oh boy, wait until you get your first set of older Cantis!)
Adjusting shifting requires the most skill of the three, and that will come with time and practice, but the more you can learn about the basic concepts early on, the better.
Good luck, OP. Also I can't stress enough that it's never a bad idea to ask for insight or help from your more experience coworkers. If you're unsure about something, don't wing it!