The trouble with the push-ups you do on your knees is that they don't prepare you for full push-ups, which are more effective at toning abs, arms, and chest.
Here's how to build up to 20 "real" push-ups fast:
Get into the plank position and lower yourself to the floor (keep head in line with spine). If you can't push back up, stay an inch off the floor for as long as you can. Drop, get into plank again, and repeat eight times (three times a week). Once you can do one full push-up, add another every week.