I transfer bitcoin to someone i don’t know ,

how could i get back the money?

You can’t.

Bitcoin transactions cannot be reversed, cancelled or undone.

[I transferred] my bitcoin to someone I really don’t know

If you have contact details for the person, you can ask them if they are willing to create a new transaction to send some money back to you.

If you only know a Bitcoin address, you cannot get contact details from that address. A bitcoin address does not identify a person, a place or a business. It is just a number, effectively a random number.