Most babies start walking unassisted sometime between 12 and 18 months; mastering this skill is the major physical milestone of their second year. Although their early attempts look very different from mature walking (they plant their feet wide apart and lurch from side to side), they pick up speed with practice, eventually learning to walk smoothly on their own.
Shelov, S. P. (Editor-in-Chief). (2004.) Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.