How do I fix my door to lock

You’ll need to replace the lock set as the button latch within the inside handle is worn or bent. Given this is an outside door you should get a deadbolt installed either within the same handle plate or independently.

The easiest way is to go to the local hardware store with a good set of pictures so you can get the same model. If you have the make and model that would be even better! You might find there is a second version with the dead bolt.

Adding the deadbolt is not overly hard if you have access to the needed tools.

Here’s a useful link: This Old House - All About Locks