Eupark, one aspect to consider is your level of fitness prior to your surgery, your age, and your present level of fitness. Work gradually to increase your general fitness... i.e., light cardio and light calisthenics with or without light weights.
Targeting only your abdominals probably isn't the best way to achieve overall fitness. Sit-ups only target a specific group of abdominal muscles, and one should include a number of different core exercises to strengthen your midsection overall. (Crunches, planks, oblique sit-ups, etc., will all help to strengthen the entire area of concern)
Yoga is an excellent form of exercise that can improve the body and the mind.
Seek assistance if you are new to these forms of fitness as a little instruction will ensure good form and help prevent injury.
It took me a few years to get my conditioning back on track after surgery, so be patient and use common sense in starting any new exercise regimen.
Good luck )