When I first starting knitting I had so much trouble learning how to purl. It took my ages to learn it and even then I kept forgetting how exactly to do it. It didn't help that I have trouble understanding written intructions some of the time, so in the end I had to get my mom (who does not knit, but can understand instructions) to help me.
Just watch them over and over. Or better yet find your LYS and when they might have knitters just sitting around knitting (tends to happen often at LYS's for some reason :D ) Seeing it in person sometimes helps. Don't worry about being slow. There is a lot you can do with just a knit stitch! As an exercise in purling once (a loooooooong time ago) I made the basic washcloth (cast on 3 or 4 stitches, K 2 rows, then K2, YO, knit to the end of the row then decrease back down) all in purl instead of knit so P2, YO, Purl to the end of the row. It really helped me gain confidence of purling. Of course I had been wrapping my yarn incorrectly for about 2 years before I realized that I was twisting my stitches. I couldn't figure out how anyone would want to purl since it was so dang tight! (insert v-8 smiley here). Once you get purling down everything else is based on those two stitches (in one crazy way or another!)