Q. How do I fix my weak upper body?
A. Strengthening your upper body is crucial to total fitness, whether you’re concerned with bone density, puny shoulders, an inability to lift heavy objects, or you just want to attain an hourglass shape.
Progressive exercise is the key to getting stronger. You must begin with exercises that are relatively easy to perform, then progress to more difficult exercises as you go stronger. Since your body will adapt to new movements within 4-6 weeks, you need constantly add new stimulus.
For the upper body, you should include at least 3 sets of 10-15 repetitions for three different movements 2-3x each week: upper body pushing, upper body pulling and core exercises. To learn more about these movements, check out this article on starting weight training.
Get my workout plan for progressive push ups -- a great way to build upper body strength -- over here.