My iphone is stolen and my Find My was not set up. I want to erase my data on my Iphone how I can do it. My phone has many personal data and informations

If you did not have Find My turned on, you may have lost your chance to trigger a remote erasure of the phone.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support

"If your missing device doesn't appear in the Find My app or at iCloud.com/find, you didn't turn on Find My. There's no other Apple service that can find, track, or flag your device for you. But there are steps that you can take to help protect your data."

Step 5 in the second article ("Remove your missing device from your list of trusted devices") is something that you wouldn't want to do if Find My was turned on, and you marked the phone as Lost. Removing a phone from your list of trusted devices would remove the Activation Lock, letting the thief, fence, or other receiver of stolen goods reset the phone and enjoy the benefits of using it.

If your phone was locked in any way (e.g., even with just the regular passcode), the criminals may contact you, claiming to be from Apple Support, and wanting to "help" you if only you give them the codes that they need to unlock the phone. If you receive such communications, don't engage with them and don't give them any codes. Just cut them off.