I love your courage and desire because it takes a determination for you to take this decision in other to work on yourself and always keep fit, so one thing you must know is that consistency matters a lot in terms of exercise because that's the only way it can work more better, so perhaps as a starter you shouldn't be too aggressive on the push up so that you will not be too stress out, so perhaps at first you should focus on how your body can withstand then on the long run depending how your body is adapting then you can increase the number of your push up and before you know you could be doing 100 push a day and trust me if you are consistent enough you will be happy with the outcome of it.