This may seem like a silly post but believe it or not, that was not something I figured out until some time later after I had owned my camera. As a noob, at first, I used to just place the lens on the camera and just move it around until I feel like it's clicking, lol.
Well, the proper way to mount your lens is to connect the dots. Your Canon lens will have either a white square or a red dot.
Step 1. Align the dot or square from the lens to the camera
Step 2. Turn the lens clockwise until it stops and you hear a click. This means that it is locked.
Step 3. To remove your lens, press on the lens release button and turn your lens counterclockwise at the same time.
With that said, you may wonder what's the difference between the red dot and the white square.
EF (Electrofocus) lenses are mainly meant to be used on Full Frame bodies and comes with the red dot, whereas as EF-S is meant for use on Crop Sensor bodies (low to mid-range) which comes with the white square. If your Canon DSLR camera comes with both a red dot and white square, it means that it is compatible with both EF and EF-S lenses.
The only disadvantage of choosing an EF-S lens is that if you plan to upgrade to a full frame body camera, aka professional camera, then you will not be able to use the EF-S lenses.
Hopefully, I have not confused you. So to summarize what I just mentioned, Canon full frame bodies are only compatible with EF lenses. Canon crop sensor bodies are compatible with both EF and EF-S lenses.