I'm 20. There are too itchy rashes on my inner thighs on both sides. After itching once I can't stop my hand until pain happens. Rashes are spreading to my testicles. What to do?

Call for a clinic or private doctor.
Meanwhile, go buy some cortisone cream, and maybe some spray too.
Wash with unscented soap, rinse with tons of water, pat dry, and lie on your bed to dry.
Then apply the cream.

That is just TEMPORARY relief if you have a fungal or bacterial infection, for which you need a nurse or doctor. Fungus of the type called 'athlete's foot' or 'jock itch' is the most common, and can be anywhere on the body, man or woman. You can talk to your pharmacist if you can't see a doctor easily. There are anti-fungal medicines available without a prescription, and you can jump right to that and skip the cortisone, but it's better to be diagnosed.