How do you know you should now masturbate?

What does it feel like to want to have sex?

I don't understand why people want sex. I've tried to masturbate but I don't feel anything.
But, now I've strange itchy feeling in my genitals. I moan when I touch them but not sure if that is what it feels like or is it just sensitive because I itch using my panty. To itch I even press my heel towards my vagina but I don't want to get any further in.

Pleas help me understand how it feels like when you are aroused.