Every outside door should be fitted with a deadbolt. The best ones for crime prevention require a key to unlock them from either side. But remember to keep the key handy from the inside so that someone can get out quickly in case of a fire. Sliding glass doors are a common entry point for burglars. Often they're behind a home and out of sight.
Tracks are often loose and allow locks to be easily defeated.
A pin inserted through a matching hole in the door and frame from the inside can be sufficient to keep the door from being jimmied. Or a piece of dowel rod can be laid in the bottom track to prevent the door from opening. Other fancier locks are also available. Remember to lock your doors and windows even when you're home. In some places one third of the robberies take place while someone is home.