Increase your ability by 1 push up everyday like, 1,2,3,4,5,6 ......... 100
Then, after three months you should be able to do 90 push ups in three sets (30 in one set and 5 to 10 minutes break between each set of push up). And off course, do warm up like jugging, jumping and stretching before doing push up.
Can any one do push up by one hand? I could do 2 push up using 1 hand.