How long were you casted and how is your ankle range of motion? Regaining any losses there will be the most important. Try moist heat followed by gentle Achilles stretching. If you're in PT you may benefit from ultrasound. Ankle range of motion exercises in NWB have probably already been started. As far as strengthening goes you need to focus on eccentric strengthening, this means the return from a contracted state (i.e., the descent portion of a heel raise) - this is what strengthens the tendon. However you may not be cleared for this so check with your surgeon.
Other exercises that will be important our towel scrunches (sit on a chair on hardwood floor, with towel on the floor curl your toes to pull towel toward you). You'll be surprised how weak these muscles will have gotten.
Tibialis posterior (a bit hard to explain, can look up videos online)
Hope this helps