To be qualified to enter the command upgrade selection process you need to have been in the company for 1 year, with 2 sim checks graded good or better. You'll then be able to apply for the selection process consisting of a sim check, interview and technical exam, and if you pass the selection you'll be put on a command course according to your seniority.
You will then be asked to bid for your 3 preferred bases and will be given your base during ground school. Generally, you will be placed wherever the company needs you and upon completion of final line check you'll be given whatever base that was specified as your 'release base'. At the next base bidding process you can then request your preferred base and again, seniority according to MSL applies as to who gets what. In short, you're entering a bit of a lottery. Some people with good seniority sometimes get to stay, but in general, expect a move to one of the less popular bases.