Try any of the online booking engines, autoeurope.co.uk, rentalcars.co.uk, etc. to locate/choose a car, google "cheap car hire in USA". Expect any car you choose to have automatic transmission and remember that they (like mostly elsewhere) drive on the right. Then there is the matter of insurance: In the USA it is common for people to be able to use the insurance on their own car to cover whenever they rent. The result is that it is usually far cheaper to separately take out insurance cover as soon as you have booked your car (some rental companies don't even offer full cover). There are several online options for this, such as insurance4carhire.co.uk, icarhireinsurance.com, etc., google "excess car hire insurance" and go for the North America option/package.
Driving in the USA is for the most part very easy because their infrastructure is so biased in favour of the private car. When you approach a stop sign at a junction, take note of who is there (on the other approaches) as you arrive. If it is a junction where everyone has a stop sign then they have a rule (written or unwritten) where you work on a "place in the queue" principle - must occasionally have at least near misses. Note that there are lots of examples of traffic lights being overhead in the middle of a junction - you really must not fail to notice these. In urban areas the traffic feels a lot "sleepier" than in Europe. Always observe speed limits. We tend to avoid toll roads for fear of being unable to pay correctly. Hope you enjoy your holiday, we found the NE USA comfortable to visit.