Use a stud finder when looking for wall studs. Don't rely on knocking. Don't hammer a nail directly into dry wall, it will come right out. Use a wall anchor if you're hanging something that isn't on a stud. Pay attention to the weight restrictions listed on the boxes of hardware, going over them could result in your things falling off the wall. If you don't have a level handy, you can use a golf ball or a glass of water to see if a shelf is level.
