Knee cap comes out of place

Im a member of the general public and after some advice.

Well from a young age i had a problem with my left knee cap, it comes out of position i was sent to physiotherapy and said that i should complete some exercises. Now and again i seem to get problems with my knee. (but dont do the exercises anymore as didnt bother me too much)

But sometimes when i come down the stairs it locks but doesnt need to be pushed back in to position.

Could anyone recommend some exercises i can do, i googled it but it said you would need to build up the muscle before surgery or building up the muscle might prevent you have to have surgery.

Im 25 now, ive been having this since i was about 9... But lately seems to bother more esp when driving. If i was to straighten my leg while sitting down i cant straighten it much before i need to stop as my knee cap then moves out of position.

I look forward to your replies.. :-)