One of the first signs of puberty in boys is bigger testicles. The next thing you're likely to notice is that your penis has started to grow and get longer. It may also get hard when you touch it or rub against something. This is called an erection.
Sometimes you can get an erection when you think of something that's sexually exciting. It can also happen for no reason at all.
Your penis and testicles will continue to grow until you get to the end of puberty.