I hit my knee hard on the frame of the door , i can walk but it hurts especially when i squat , what should i do , and whats happening to my knee ? thank you?

If there is no significant swelling or joint deformity, your knee should be fine. Most people feels discomfort with squatting then standing up after a minor knee trauma but it should resolve completely in a couple days. You can apply ice pack to the knee if there is mild local swelling. If the symptom does not resolve completely after 7-10 days which I doubt, you can see an orthopedist by then.