Uk Citizen Marrying A Us Citizen

Hi, i am a uk citizen and i met my girlfriend in the us, i have now come back over and we are planning on getting married very soon. We are both at a loss as to what to do first, what we need, how much it will cost and basically everything about it. If you have any info or know any websites that will help me or explain something that would be great because i have had no luck as of yet finding any!!
Thanks ben