Get an instructor to take you through the quite different approach to airspace, flight plans, etc.
ATC speech takes getting used to, quite why they have to talk in such a shotgun manner is beyond many of us. You will also find controllers who are pleasant and helpful, and some who are the absolute opposite. It doesn't depend on workload.
Sharing major airports and airspace with large commercial and military jets can be a surprise.
Circuits (patterns) can be far tighter than what you are used to.
Magnetic deviation in the western US is very substantial, comparing compass with chart. There are also some items (I forget which - weather ?) that give times in GMT, which is no problem in the UK but requires quick thinking over there.
The practiced forced landing into convenient nice green fields of the UK doesn't extend across much of the US where the terrain is almost entirely hostile. Country roads may seem to offer an alternative but check for telephone poles alongside. You may be reduced to a lake as a final option.