Rolling is the first way your baby starts to move around. Some babies will roll across the room to get to a toy, but most babies will roll around for fun and only 1-2 rolls.
This means crawling is the first way for most babies to get to a toy by themselves.
Some babies will belly crawl before they push up on hands and knees. Some babies will skip belly crawling and practice crawling with a plunge forward. Some babies will crawl backwards before they crawl forward.
When a baby is crawling he holds his head up to 90 degrees, he pushes up on straight arms and pulls his legs under his belly. He then holds his trunk above the ground, and he maintains all of the above while he shifts his weight moving forward.