You'll pick whatever bases you want in order of preference when you start at the company. Say: 1 PHX, 2 LAX, 3 SEA. However, just because you request it doesn't mean you get it. That's determined by your seniority (which when you're new is right at the bottom). You'll then get "awarded" a base based off your preferences and what your seniority can hold. Each month when there's a vacancy or realignment bid, you can update those preferences and try to snag a different base (or CA seat if there's upgrades).

You'll be on reserve in whatever base you are in until your seniority lets you hold a line. This usually occurs when you bid about 80% in base (have 20% of the pilots in that base below you in seniority). Just because you've hit 80% seniority in base doesn't mean you have to have a line. You can bid down to reserve if you want to make sure you get the days off you want (a useful tool, especially if you live in base, much less so if you're commuting). But everything is seniority driven, which is why seniority is everything. Also, being in the "junior" base helps you get better seniority in base faster than a "senior" base and will affect your reserve time before you can hold a line. Hopefully that helps!