Ideally, you want to find the studs inside a wall, and attach your desk
using a course threaded lag screw of adequate length right to the stud.
If there isn't a stud in the wall (unusual), then you can use a toggle
bolt. If you're not familiar with them ask someone at your local
hardware store how they work. I use them all the time. You'll need to
drill a hole in the wall to insert the toggle, and use a fender washer
(or large washer) by the head of the bolt . . . an easy solution!