How long does it take you get pregnant on provera and clomid?

I'm taking provrea because my periods r inregular and it's Been that way sense I started I'm really scared that I might be infertile could this be true. My doc said I wasn't but I think she pulling my legg.do u think clomid will be something to look into! I'm only 20 but I don't want to wait to long to get pregnant!! And is True that when ur under 25 its a good chance of twins!!! Please help thank u