to be with my girlfriend

and i have no idea where to even start im going over next year for 4 weeks
but i understand i need a greencard or i cant work
so any help and advice on what i need to do to get the ball rolling on this
would be great
here is some info on me
im 34
uk citizen i was born here
im self employed trying to get my own biz up and running
hope that helps
thank you for any advice