How can I unlock my iPhone?

Contact the carrier it's locked to and ask them to unlock your iPhone from their network.

If you don't know the carrier, contact Apple Support and ask.

Also take a look at the article below:

How to unlock your iPhone for use with a different carrier - Apple Support

Can you tell us how is it locked so we could provide you with the steps?

Carrier locked?

Activation locked?

Disabled?

Or what?

What do you mean?