It is hard to give general advice without knowing specifics – did you do Pilates before the surgery? Are you asking for mat or equipment exercises? What surgery did you have? Have you been cleared by your surgeon to resume exercise? Are you already doing physical therapy? Any movements that are contra-indicated? How are you weight-bearing on your knee (like on all fours)? In general you want to work on strengthening your quads (especially the inner portion), adductors, hip abductors, glutes, and hamstrings while maintaining enough flexibility that movement isn’t limited.

All the exercises I mention above are safe for you, so I would start there.