There many manufacturers of sliding doors, there are also many different types of locking mechanisims supplied with these doors, many are very good and some are not so good or secure. You might want to have a professional locksmith check yours out.
If you are a "Do It Yourselfer" most of the products listed on this page can be installed or replaced fairly easily.
If you want to be able to unlock and enter from the outside be sure to choose a lock with a keyway cylinder on the outside, you may be able to have a professional locksmith rekey your cylinder to match the other doors in your home. Your should not include any items that only operate from inside on these doors.
If you do not need to unlock and enter from the outside many items are available that attach to the back edge of the sliding door and project a bolt into the upper or lower track or even the bottom of the stationary door. Some of these items use a key lock, which may be a danger in case of fire or other emergency.
Manufactured blocking bars are also available the wedge against the back of sliding door and the door frame, an alternative to these items could be a wood dowel or piece of wood.