How to hang thes shelves so they don’t tilt forward.

Use screw in wall anchors in walls. Screws go in the anchors and ajdust in or out. The key hole slots slip over the screws.

Add a "bump" of hot glue to to bottom of the back of the shelf. That is, below the keyholes.This will push the bottom of the shelf out a bit from the wall which should keep the shelves from dumping things on the floor. The size bump might have to be a guess, but if you hold the shelf out to where it's level, it'll give you an idea. MY guess is that the bump should be about as thick as the keyholes' or sawtooth's protrusions...

M.M. has the right idea. You can also get "glue dots" if you don't want to do the glue gun, which can be a bit messy. Also a strip of thin molding works.

If your wall where you want to hang the shelves is not completely level, then the shelf may continue to tilt forward, especially when something is put on it. Keep trying the above suggestions with the silicone dots. I have also used thin popsicle type sticks as shims. With patience, you should be able to get the shelf to sit up and tight against the wall.

Also, if the screws aren't tight enough the shelf will sag forward. You have to leave them out a little to go into the keyhole slot. Just keep tightening the screwh a tiny bit at a time until it fits in that keyhole slot nice and tight. You'll have to remove the shelf, tighten each screw a little, put the shelf back up over and over until you get it just right. Good luck!