The first change that is usually seen in puberty is an increase in the size of the testes. After the testes begin to enlarge, the penis will start to grow in size and length. Testosterone is the hormone that contributes to the growth of the penis, and it is produced by the growing testes. In adult men, the average penis length is about 5 inches (stretched out), but can range from 3 + to 6 + inches.
Hair under the arms and in the pubic area starts at about the same time as the testes start to grow, but the whole process isn't complete for years. The fact that you have some hair under your arms indicates that some pubertal changes have started, which is reassuring. You should also notice that your testes have gotten larger. If they have not, then it would be a good idea for you to ask your parents to take you to see your doctor. Your doctor can examine you and talk with you further about whether your puberty is proceeding normally.