Read the contract from beginning to end, don't just flick over it.
Read about how you submit your invoice, what information they want you to quote - order number - job number etc.. if you get it wrong you will wait ages for your money.
Confirm who and where you send your invoices to. get a job sheet signed off when you complete the work especially if it is a satalite branch and you are sending invoices to head offices.
Trev