The inside of my left knee hurts whenever i have to squat down, or stand up from sitting. what could be going on? how do i fix it?

Medial knee pain: Can be a sign of inflammation, or possibly a strain of the ligament. Your specific complaints tend to be concerning for a meniscus injury. You are too young for significant arthritis symptoms. That being said many people can have medial knee pain for various reasons. Typically I would treat the symptoms with relative rest ICE and Nsaids and see if it gets better. If not have it evaluated.