He's off! Crawling is the one physical skill that can wildly vary in attainment - and some babies never crawl, preferring instead to go directly from sitting to walking. It's also normal for some babies to crawl as late as 12 months, although most start somewhere between seven and ten months.
To crawl in the classic hands and knees fashion, your baby has to first be able to sit well on his own and have good co-ordination, muscle strength and control. You may find that he rocks back and forth on all-fours for a week or two before he starts to get going - this action allows him to learn to co-ordinate his limbs and to work out how to get his body to go forwards rather than backwards (some babies crawl backwards for a short while too).
Before she truly gets moving, ensure that you thoroughly baby-proof her world - and this means more than checking your kitchen, living room and the bedrooms to ensure that she is safe at all times. Aside from doing a general safety check of the house - ensuring that you have smoke detectors and fire extinguishers in place - make sure that you check all water areas, the front and back gardens, verandahs, the car, and any other places (such as grandma's house) where your baby spends regular time in.
While it may be tempting to invest in a baby walker - a sling that's suspended in a rigid frame on wheels - to give your baby extra mobility, almost all experts agree that baby walkers are not a great idea. Not only can they be dangerous - there have been a number of nasty accidents involving baby walkers where the baby has 'walked'' down stairs or tipped over on an uneven surface - but the way that babies manoeuvre themselves in a baby walker, using their toes and the balls of their feet, actually hinders their physical development so that walking can be delayed.