Learning to Roll Over
Your baby's first step toward mobility isn't actually a step at all -- it's a roll.
When she's about 2 or 3 months old, she may flip from front to back. During tummy time (which you should supervise), your baby may lift herself into a push-up position and then start to rock back and forth or kick her feet. Then, if she's strong enough, those movements will send her rolling over. (She may get startled and cry the first time!)
Flipping from back to front often takes until around 5 months because it requires more coordination and strength. At that point, some babies take up rolling as a means of rapid transport. You don't need to coach your baby to roll, though; just make sure she has a safe place to try it out if she wants to.
Since you can't predict when your baby might take up rolling, keep a hand on her during diaper changes and never leave her unattended on an elevated surface.