To hang heavier items on your walls, you’ll need the solid backing of a stud, that’s one of the vertical two-by-fours behind finished walls. The most reliable way to find them is with an electronic stud finder, which detects changes in density. You pass it over a wall, and lights and tones will tell when you’ve found the stud.
You can also hold a lamp light close to a wall, and look for nail heads and seams in the drywall. Other low-tech methods include tapping and listening for hollow spaces, or looking alongside switches and outlet boxes, since they are usually attached to the side of a stud.
The good news is that once you find one stud, you should be able to find them all, because most walls are laid out with a stud every 16 inches.