All babies are different and the age at which they start to crawl can be as young as 6 months or as late as 12 months.
Generally, the baby starts with the 'worm'. They keep their tummy on the floor and use the arms to slide through the room. Sometimes children who are used to a slippery floor, such as laminate, may skip real crawling and go straight from sliding to standing.
Most children, after rolling, first crawl on all fours with the belly off the ground. When they have just started crawling, you often see baby's buttocks rocking back and forth in the air. Sometimes baby may begin by crawling backwards until they learn how to move forward. Once they have learned to crawl properly, they'll race through the house at a wondrous pace.
Babies love crawling through the house at great speed, exploring where they haven't been yet. You now have to make sure it is safe, and baby doesn't understand "no" just yet.
It is difficult to estimate what could be dangerous for a crawling baby when you are just standing up. Go and crawl through your living room - what's dangerous? Try to look through the eyes of your child.