If you are just starting, you should do about 20 push ups, around 20 squats, and 20 sit ups, a day, this will help you get bigger muscles, better abs, and better physical stregnt, do remmember after about a week of this, you will have to up the game, and increase the amount you do.
Do your push ups at different angles, do them at different speeds
Try to do them as an intervals
Do as many as you can in 20 seconds
rest 10 seconds
repeat this 8 times for a total of 4 minutes
Don't do them everyday
do them 3 x per week
Find a bar or a tree limb and do some chin ups or pull ups
If you are not sure how to do High Intensity Intervals then I recommend visiting http://www.zenmyfitness.com
and download free workouts and nutrition plans that explains the above
In order to build strength you need to develop all the large muscle groups in the body. Focussing on a certain exercise for a certain muscle group is a fast way to getting a unusual look. Push ups are good but you may need to bench press a heavier weight for 6-10 repetitions, do this exercise once a week. Second training day you will need to work your back muscles by doing wide angle chin ups for 6 reps and a low row for 6 reps always do at least 3 sets. Third training day is weighted squats in the same fashion as the other exercises i e 6 reps 3 sets. Make sure you stretch before and after your workout and you will see moderate results as you are only 15.
you have to do atleast 200 every OTHER day