Hi Mahdikooshkakie,
I would recommend to follow this article:
Get help with Personal Hotspot on your iPhone, iPad, or iPod touch - Apple Support
Also remember that the personal hotspot is a feature that is controlled by your cell phone provider. If you are not able to use your personal hotspot and your software is up to date then it is most likely a problem with either your cellular account with your carrier or a account issue on your sim card.