I'm within my first year in the trade so I can help guide you on some of these. I still am not 100% is we're allowed to talk about per diem and pay since this isn't a union site so I'll err with caution and begin with check your state labor and industries website or google "Georgia state apprenticeship programs" and see if your state has a list with the booklets explaining each apprenticeship.

1) From speaking to mechanics if your in Construction there will be times of layoffs seems to be every 4-6 maybe a little longer years or so. That's in Seattle.

2) Getting in is the most important. Work for whoever you get called to work for. Ups and downs to both. I work for a big company. Lots of work, lots of construction, lots of repetition. But it's good to start there, you learn how to put everything together and how it works and why it works (if you ask the right questions) Some guys getting in around me work for a small company and they do adjusting and mod jobs. Still have a lot of work, is it guaranteed for as long? I couldn't tell ya. More availability to move around the states with a big company also. Easier to transfer.

3) If you want OT, Construction is where you want to be. 7 days a week is not unheard of. Construction will be cold, wet, hot, wet, dry, hours could range from 4-10s to 7-? the sky is the limit. Service, mod, repair. Working in existing, running buildings. Can only make noise at certain times of day, hot - you're in a heat tunnel. more regulated schedule 4-10s, 5-8s.

4) Ill refer you to your state apprenticeship site.

5) Mileage/Zone/Per diem. Mileage is if you have to travel between job sites or destinations during the day in your vehicle./ Zone is to help pay you for traveling within a certain amount of miles from a set location. SO kind of a "set mileage if you work in here" pay./Per diem is a pay for working outside the furthest zone set. Refer to your local page and/or state site.

6) Listen - You will learn things that you're not able to comprehend when you're being asked to do them. So follow instructions and listen to your mechanic.
Ask Questions at appropriate times - Goes along with first - Ask after the task is complete before starting something new, or at break, etc. Learn when your mechanic wants these
questions.
Channel Locks, Tape measure, 11-in-1 Screwdriver, Pocket Knife, Sharpie, Pencil, Notepad. Bring all of these, your mechanic will supply the rest.
Leave your phone in your pocket.
Hard work, showing up early, cleaning when down time. Just be a good worker and able to learn.

Good luck