Whats the best way to strengthen my leg after knee surgery? After not using my leg its weak and I have a lot of atrophy..

Physical Therapy. A professional therapist can target the areas that need improvement without increasing the danger of further injury.
See this. After a joint replacement, you can check: http://orthoinfo.Aaos.Org/topic.Cfm?Topic=a00301 after knee arthroscopy, you can check: http://orthoinfo.Aaos.Org/topic.Cfm?Topic=a00300.