how can i hold my breath for longer while swimming?

I think it is like developing stamina and endurance for anything, like running, for instance. No one can start running a marathon of the first day of training.  The runner needs to work up to the point of being able to run a mile, then 2 miles, and gradually work up to 5-10 miles running a race.  It takes time and I imagine that to learn to hold your breath under water for any length of time, it will take practice.  I know the more you do it, the longer you can hold your breath.  Just be careful not to inadvertantly pass out from holding it too long! Just keep practicing... actually, if you are holding your breath for a minute, that is pretty good, in my opinion.