Hey there...Like most activities you have to work up to it gradually. For push ups, start with the normal position but with your knees on the floor. Try doing 5 reps and stop. As long as you do this every other day, u can build up to 10 reps. Then try to get up to 3 sets of 5 reps staying on your knees. Keep at it and eventually you'll be able to go onto your toes. Core work is best done with "planking" and other similar plyometrics. Good luck !