Marrying My American Boyfriend

I intend to travel back to amercia next month and stay for the maximum 90 days - on the visa waiver programme. During this time i want to marry my american boyfriend. If we go ahead and marry what will i have to do to ensure i will be able to stay beyond the 90 days ? if i travel back to the UK when my 90 days is up will i be able to travel back into the States or is it likely will i be turned around at immigration? I want to do things correctly but have been told there is a 6 month wait for the K1 visa and we don`t want to be apart any longer than we already have.