So much depends on the local, then the dept. you are hired into. Local 19 has a state that allows marijuana, so we are having a giant issue with candidates unable to pass simple UA's.

Best advice, already stated here and require zero skill:

Be early, 15 min. Should do.
Attitude- never have it.
Energy - be ready to work! This trade is a damn privledge, not a right!
Be prepared - still use this, what's an apprentices job description? To make his or her mechanic look good.

Items that should be on your person each day include:

Notebook and pen/pencil
Channel locks
10-1 screw driver
Tape measure
Electrical tape

With those simple items you can do a surprisingly large amount of tasks. Again each dept. may have a few different items to add in, such as service repair may want you to have 1/8th inch tweaker and door pick on you as well.

Nobody expects a new hire to come in knowing anything, so don't pretend to. Effort and attitude will get you as far as you ever wanted to go. I will spend years training the right sort of person, won't spend 15 minutes with the wrong kind. Good luck, work safe, and work within the agreement.