For the PSATs, there are three levels you can reach:
I don't know much about being commended, but if you make the cutoff score for being a semi-finalist, they'll notify you around August or later of your senior year. The cutoff score depends on the year and the state. I think the cutoffs range from around 210 to 223.
If you become a semi-finalist, then you'll have to do some additional stuff, including writing a short essay. You'll be notified around March of your senior year if you become a finalist. From then on you can get some scholarship money.