I couldn't do push ups too when i started my Diet and Fitness , all i can recommend for a fast progress is This Book " Men's Health Your Body is Your Barbell: No Gym. Just Gravity. Build a Leaner, Stronger, More Muscular You in 28 Days!" this Book is really helpful and from the title it's all about body weight training and you will find a whole section of the book about "Push-Ups" from a person who can't do a single straight push up to someone who do the Plyo-Push ups really easy ^^ , you can download it from Torrent , purchase it or you can get it from any place who sells it near you it's the absolute help .
The Book Link on Amazon ==> http://www.amazon.com/Mens-Health-Your-Body-Barbell/dp/1623363837/ref=sr_1_1?s=books&ie=UTF8&qid=1434500831&sr=1-1&keywords=your+body+is+your+barbell