Yellow stains in the armpits usually arise from a chemical reaction between sweat and deodorants containing aluminum. Unfortunately, they are quite common and difficult to treat. The best thing you can do to prevent them from ever emerging is to rinse off the armpits of your shirt after use, or hand wash them directly with A+ liquid detergent. If you put a garment of untreated stains in the laundry basket, the stains can stick harder to the fabric, making them tougher to remove.
For the best chance of getting rid of the stains, pre-treat them by gently rubbing the A+ liquid detergent into the stain. Leave the detergent on and then wash the garment as instructed. To prevent staining, it is a good idea to choose a deodorant without aluminum, and to use only a thin layer. Also, wait until the deodorant has dried on the skin before putting on the shirt.