Push up and sit up tips

When you do push up and sit up, make sure that you are not wasting your time and energy for nothing. Most of the people tend to do sit up with their back, not their ab ( abdomen ) muscle, their back muscle takes the main part in the effort to lift the body. This would be very exhausting for you without any good result for the ab shape. The right thing to do is make sure that your neck and back muscle in relax condition, try to work your ab muscle ( slowly but sure ) , concentrate to lift your upper body with that muscle, may be you can only lift it a bit, but don't worry, that's fine.
You will have your ab muscle very tight and a bit cramping, that's the good sign, it works !
For the beginner, 3 reps ( repetition ) would be appropriate, but just keep in mind, 3 good quality reps is much much better than 30 bad quality reps ( although you do it with a lot of sweating and grunting, ooooughhhh ).
Well, have a nice try, I will come back with some push up tips.