Starting Birth Control

I’m 23 and not sexually active. I have not had a period in 5 months and recently saw my dr. To talk about what was going on (I have always been somewhat irregular and thought this was due to stress). After bloodwork returning elevated testosterone and prolactin levels, PCOS has been a talking point. I took birth control when I was in high school and experienced breakthrough bleeding that lasted 3 months.

All that to say, I am starting birth control AGAIN next week. What can I expect as far as side effects, mood swings, etc.?