Why don't you want her to suck her thumb?
I have one kid who is a thumb sucker and another who's a pacifier kid. I'd choose the thumb sucker any day. He can't ever lose it, it does a great job of soothing him.
Babies have a natural suckling reflex which partly soothes them. One problem with pacifiers is that they can be lost, and if the baby can't find it again or if it's completely lost, you're stuck without something to help soothe them.
Our son's 3.5 and only sucks it when he's tired. He used to all the time, but now it's only a soothing mechanism at bed time. Both the pediatrician and dentist agree that he'll likely stop before he's in kindergarten. It's actually be a Godsend when he fell and pushed one of his teeth back significantly. The thumb sucking pulled it back in line.
You won't be able to force a kid to choose one or another as young as she is. She may not like the size or the texture of the pacifier all of a sudden. You might want to get a few to see if there's a brand she likes better than others. If you are breastfeeding, it is really difficult to replicate the shape of your nipple in a pacifier - I honestly believe that's how our son got attached to his thumb.
At that age, our daughter was sucking her thumb, but she abandoned it for a pacifier when I started chemo and had to stop nursing. Like I said earlier, of the two vices, I'd much rather have a thumb sucker than have to deal with getting rid of the pacifier.
Good luck and enjoy every moment with your baby.