I had a knee injury in high-school. I tore about seven ligments in knee. Sometimes it feels like my knee is giving. What can I do to strengtligimennt?

Ligaments. If you had several ligaments torn in your knee and did not have surgery it is unlikely that you will regain stability of the knee. You can try to strengthen the muscles about the knee with a weight training program but this will provide stability.
Get evaluated. See an orthopaedic surgeon and have your knee examined for a proper current diagnosis. Physical therapy may be of benefit.