Before you start with lower body resistance training at gym, consult physiotherapist to confirm whether you have recovered full strength in your knee muscles. If you are having pain during knee bending, let physiotherapist check whether your thigh muscles are tight. If they are tight, stretching will help to resolve the pain. Also since you feel instability while kicking ball, you will need to undergo physiotherapy treatment to practice exercises in weight bearing position to improve your knee stability. After you ascertain about your knee stability, you may start playing soccer or any outdoor sports. Once you complete proper rehabilitation for your knee, you will be able to play outdoor sports like before.