Hi and welcome!
We travel from Germany, not the UK. I would expect that you would be entering the us under the "visa waiver program". If so, please take note that you must pre register. Here's the link. https://esta.cbp.dhs.gov/esta/
once that's all approved and you are flying towards the us-you'll have to fill out two short forms on the plane-one for customs and one for immigration (I 94). They will want an address-so make sure you have a street address (hotel, friends, whatever) here's the link -http://www.cbp.gov/xp/cgov/travel/id_visa/i-94_instructions/
If you have a layover and a connection-make sure you have minimum 2 hours before your connection. with JFK or PHL i would try for a 3 hour layover. It is expected that the "sequester" nonsense in the us will be affecting staffing at both immigration and customs very soon.
there are two lines at immigration-us citizens and legal residents- and visitors-our experience is the citizen line is faster but not an option to you. Do not try to make a shortcut here, you must stay in the visitor line. it's a small thing but they will send you to the other line. Here in Germany i often take the shorter line, and have no problem-very different in america.
you present your papers and passport first to immigration, they ask you a few questions, get i believe your finger prints and biometric eye scan. If you have no substantial history of multiple entries to the us, you "should" have no real problem. it's just a matter of going through the process. Generally the problems my partner has encountered are due to multiple entries over the years. they will want to know what is your purpose in coming-vacation is a great answer. As an american i even get questioned.
Next your off to collect your luggage and go through customs. usually very quick-don't bring meat though. they are super picky.
once through you re check your bag to your destination, often go through security again (even though you have not left a secure area) and on to your next flight. flying in america one must remove shoes/belts etc before going through the scanning (unlike germany). you can opt out of the full body scan and get the pat down. this generally takes no more than an additional 5 minutes.
when planning our flights, i try to avoid both jfk and phl. i prefer to plan our us entry through detroit. they are reasonably quick. same with DFW.
hope that helps, any other questions just ask. i can only say it is so so worth the trip-there's nothing like michigan. i'm counting the days and waiting..........
make the trip! bring a friend! bring 2!!