i bought my phone in america and is locked therefore i wanted to unlock it so i can use it with UK sim card

You'll need to contact the carrier it is locked to in america, only they can unlock it.

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

Hopefully you are aware that any warranty issues with your phone will require that you return to the US or send it to a trusted friend. Apple warranties are not international. Also, some carriers will only unlock phones if you have a current account with them. I'm not sure how long ago you purchased the phone but you may want to look at the possibility of returning it and buying one in the UK.