It's perfectly normal for the skin on your penis to be darker than the rest of your skin. The texture of your penile skin is also different.

During puberty, certain changes occur in human genitalia due to hormonal changes. For both males and females, the genitals grow much larger, skin takes on a more wrinkled texture, and the pigmentation in the skin of one's genitals darkens up to some degree. Some people (men and women alike) get darker than others down there.... just like some people grow more or less pubic hair.

Just take a look all over this website and you'll see.... you have nothing to worry about.