My best advice would be to wait until she's older. My boys were both 3 1/2 before they were potty trained (ok, closer to 4). I've heard many people say that girls will train at a younger age, but most M.'s I've talked to waited until closer to 3.
If you do decide to wait, it will often happen just like that. My oldest trained in just a few days. My youngest took a couple of weeks.
The plastic training pants are meant to be put over the underwear. This is so the child is wearing underwear and feels wet or poopy and learns from that feeling (or learns to dislike that feeling), but the plastic helps the mess be a little easier to clean up. I didn't use them, but I'm sure some parents do and appreciate the mess being easier to clean up.