Don't worry. Relax. Its easy.
When you arrive at Heathrow, and get off the plane, there will be two ways to go, both clearly sign posted. One will be arrivals tro the UK. The other will be for connecting/transfer flights.
Follow the signs for transfers. You should be able to arrange to have your luggage auto transfered when you get onto the plane in the US.
You then just need to find the gate and departure time of your new flight - it should be from the same terminal (International flights). Then its just a case of waiting around.
As you are not leaving the airport, so not "really" setting foot in the UK, you won't need to go through immigration control.
It is very easy to do, and there are lots of staff on hand to help you if you are not sure what to do.
They certainly won't let you go anywhere you are not supposed to, and if you can't find the correct gate in time, they'll send someone out to look for you.