What can I do if I have patella chondromalacia?

You have to be seen. It meant "soft cartilage under the knee cap", a presumed cause of pain at the front and especially inner side of the knee. Treatment range between rice (rest, ice, compression, elevation), anti-inflammatory medications, and physiotherapy. Treatment with surgery could be done if other methods failed.
Therapy. Chondromalacia patella (runners knee) is due to overload of the patellofemoral joint (knee cap against the end of the thigh bone). Running causes high forces in this portion of the joint. So does arising from a chair, going up/down stairs. Treatment consists of activity modification, ice, anti-inflammatory meds and therapy aimed at flexibility and strengthening, especially of your core.
Patella femoral . If your pain is in front/under your knee cap, then most likely patella femoral pain syndrome/chondromalacia patella. Best managed with exercise to strengthen your quads to improve tracking, and avoiding deep knee bending activities. Also oral or topical nsaids can be of benefit, as well as a brace. Injection of cortisone or hyaluronic acid also very popular. If all else fails, then scope.