To make the transition, consider a toddler bed, which is the same size as a crib and uses a crib mattress. It's a safer option than an adult bed; small children left alone in a large bed could become trapped between the mattress and the wall or in the headboards or bed frames.
But adult-size beds can also be safe if you follow these guidelines:
See that the bed is sturdy. Joints should be firmly in place; also check for loose nuts and bolts -- especially if your child likes to jump on the bed.
Keep it low to the ground (no more than two feet off the floor).
Place guardrails along the length of the bed, and put a rug on the floor next to it to cushion falls.
Don't place it next to a window; a child bouncing on the mattress could fall through. And avoid heating units, lamps, draperies, and blind cords.
Make room on either side of the bed so your child doesn't get trapped. But it's fine to put the bed's head against a wall.