As soon as a baby has teeth, plaque will start to build up on them. The most effective way to clean baby's teeth is by wiping them with a clean, soft cloth (if she doesn't bite you). Otherwise try a rubber brush you place on your finger or a baby toothbrush. Toothpaste isn't necessary. In fact, sometimes it hinders the process because babies will close their mouths to taste it.
Also, fluoride toothpaste is a no-no in baby care until your child is old enough to understand they need to spit it out and mustn't swallow it. This understanding comes by around age 3.