someone stole my iphone they also know my password how can lock my iphone ???

they know your Apple ID password? or your iPhone passcode?

go here and follow the instructions

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

abdulkarim147 Said:

"someone stole my iphone they also know my password how can lock my iphone ???: i need help to lock my iphone"

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Modifying your Credentials:

Consider modifying your credentials, ASAP, starting with a new password.

• Modify: your login credentials (using both a New eMail and a New Password) for Apple and each and all sites
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID PayPal and for all other accounts that have such an applicable option