I am just about to turn 12. I have some hair under my arms, but it is blonde and hard to see. It's also not very thick. I was wondering if I have hit puberty yet?
Puberty is technically the time when your body begins to change from a child to an adult. Hair in the armpits is one of those things that adults have which children do not, so yes, not only have you reach puberty, you are past it and in your adolescent years. Adolescence is the years of change between having a child's body and having an adult's body. It doesn't happen all at once, but it comes in a series of small spurts.
A doctor, many years ago, divided the sequence of changes into five stages. Stage 1 is your childhood body and stage 5 is your adult body. Adolescence is the three stages in between. Most boys don't notice stage 2 because the changes are subtle, but are distinctly aware of the changes in stage 3 because they are so dramatic. My guess is that you are at the end of stage 2 and about to enter stage 3. Not only are there hairs under your arm, but you should also see fine hairs on your groin near the base of your penis. Puberty actually started a year to a year and a half ago. At 12 you are at the average age for development. To estimate what stage of development you are in, see the Tanner Stage Calculator for Boys.
In the next several months the hairs under your arm and on your groin will both spread and get thicker as they mature. It is also likely that the hairs will darken over time. As the hairs lengthen, they will also get very curly. If you haven't experienced it yet, you will soon have spontaneous erections where your penis will stiffen and get larger in size. The erections will be accompanied by some strange feelings that will both confuse and embarrass you. Don't let it alarm you, every male goes through the same thing. Your brain is rewiring itself so you will be able to have sex when you are an adult and get married. But as the changes are made it causes odd things and odd thoughts to happen -- often at the most inappropriate times.
In about a year to a year and a half, you will enter your growth spurt when your body changes size dramatically. You will know it is coming up when your feet suddenly becomes larger and you need new shoes. Your hands also will become larger. If you look in a mirror, it will seem like you are out of proportion with your feet and hands being too big for your body size. Again, don't worry, the rest of your body will soon catch up. Now your parents might worry. They have to figure out how to pay for all the new clothes you will need for your changing body size and how to feed the bottomless pit you will develop as you are fueling that growth. But they are adults. They can handle it.