* It will grow back. Hair grows back. Shaving doesn't change the fundamental way your hair grows.
* It won't grow back thicker or darker. This is a common myth. Shaving cuts the hair off at the surface, so the new hair that grows in might *feel* a bit stubblier initially because it has a blunt end. It's not actually thicker. The color also won't change.
* It might feel different. As mentioned above, the very first bit of hair that grows back might feel a little coarser or more noticeable because it's not tapered like the original hair. But after that first bit grows out a bit, it will feel normal again.
* Your arm hair is still developing. As a 13-year-old boy, you're going through puberty. Your body hair is still developing. It's possible that over the next few years, your arm hair will naturally get thicker or darker anyway, regardless of whether you shaved a small part. This is just part of growing up.
* Be careful When shaving be extra careful, you don't want to cut yourself.
In summary: Don't worry. The small patch of hair you shaved off will grow back normally. It's not going to be a big deal.