I bought an LG optimus F6 and i want to unlock it, but it seems i got to pay for the unlock code. ¿Is there any way i can get an unlock code for free?

Pretty sure your carrier is the only way to do it.

There are ways to do it outside of your carrier, they're often shady and don't always work, sometimes they are not even permanent unlocks if your phone is reset and the profile gets reset too.

Going through your carrier is the only legitimate means of unlocking your device and keeping it that way, some carriers charge though, some may not if you put in the request and you're in a contract.

Check xda developers forums

Also, a lot of carriers won't give you an unlock code unless you've been with them a specific amount of time.