daveandg
Congrats on passing the selection and making it in to the pool!
The simple answer is that your bidding power initially will be zero. To start with you will be rostered blind lines (the company chooses your roster). Typically this will be a mix of 2/3 day trips and day trips. Longer trips of 4 or 5 days tend to be on trip lines.
The good news is that such is the pace of recruitment that you will soon find yourself able to achieve trip lines. Once you are about 10% off the bottom of your status you should start getting something you bid for. You might need to put in 30 or 40 lines to start with and be happy getting your 25th choice, but again, if recruitment keeps up you will soon move up the list. To give you an idea, in 5 years I've moved up 85% off the bottom, so now get my first choice every month. To consistently achieve 4 or 5 day trips you need to be quite senior as they tend to go to commuters who prefer to bunch their work together.