The Reverse Plank or Purvottanasa is excellent to build strength in your arms and also in the shoulders. Basically you form an inclined plane with your body being supported by the arms, with the chest/trunk facing up. The other benefit of this asana is building strength in the upper legs as they support some of the weight of the middle. After you form this posture, make sure you can breathe well as in any asana. This means, you need to loosen/relax your chest, so when you inhale you can fully utilize the top of the lungs.
To start with, you may not be able to raise the hips high off the ground. So you need to start with the little you can, hold it for a few seconds and go back to the mat. Once your arms develop strength to hold the weight, then you can focus on lifting the hips relatively higher. Your aim should be to hold the asana for eight complete (yogic) breaths. Don’t drop your head all the way to the back, but try holding it up. The more you breathe well, the more relaxed and aware you become and you will be able to figure out which parts of the body you need to work on.