I'm worried because I feel like it is a sin when I hear people telling me they "masturbate." How do you know when you are old enough? Are there any signals?
As you change from a child's body to a man's body, there is going to come a time when you start producing semen. Semen is the fluid that contains sperm and it is what can get your wife pregnant and the two of you can have children.
Once stated, the body constant produces semen in two glands called the seminal vesicles. Those two glands can only store so much semen before there is a need to release some of the excess to make room for fresher batches of semen. Therefore, once your body develops to the point that semen is produced, sometime thereafter you will experience an ejaculation. That ejaculation may come as a wet dream, where you release semen in your sleep, or it may come at some other time, such as when you are washing.
Masturbating is purposely triggering a release of your semen by touching your genitals. You don't need to try to make an ejaculation happen, it will happen in its own good time. Almost all males have masturbated at some point in their life. Some do it from curiosity. Others do it from instinct because their bodies don't release semen in wet dreams. Still others get caught up in the sin of pornography and masturbate to the thoughts of committing sexual sins. It is the latter that you need to avoid because it is highly addictive. Pornography is a form of lust, and lust is a sin. "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28).
While I know you are curious, what I would like you to do is not pursue masturbation. If it happens, then don't get upset -- it is a normal male function. But it is in the pursuit of pleasure that people can get caught up in gaining that pleasure in a sinful way. I would rather that you didn't get caught in that trap.