You have been given some great advice by DickyD and Pjh on driving in the U.S. so I don't think there is much more to add.
I have driven extensively in the U.S. over the years but all my thousands of miles have been on the East Coast between Boston and Washington D.C. rather than the West coast.
If I can add one small thing to the stuff that has already been said, one of the minor things I have found that hampers U.K. drivers to the U.S. is our mind set in always returning to the nearside lane as soon as we possibly can. We are conditioned as responsible drivers to do that in the U.K.
However on 2 lanes approaching traffic lights.....as PJH pointed out......and being in the nearside lane.....you can hold up traffic who have the authority to turn right.....against the red....if safe to do so.
A minor point I know but one I quickly learnt many years ago. So my 2 pennies worth is that although it might go against the grain, try and approach most traffic light in the outer of the 2 lanes.. Don't think there is so much aggression in the U.S on the roads and folks are generally more forgiving. They are on the East Coast I think apart from Boston ! where on my first attempt to drive out of the city centre, I made a minor error and was told " hey Buddy, if you cannot drive then get someone who can "
May I just tell you one story about driving in the U.S. and American cops. In 2008 we were attempting to drive back to Logan airport after a road trip in New England. We had done that for some years before but for some inexplicable reason I missed the ramp to the airport. It was rush hour, about 6 pm and the roads were absolutely manic. This was when the " Big Dig " was taking place !We were on a 4 lane highway and we could see Logan on the opposite side of the turnpike.
We thought our only option was to follow the flow out of the city for a few miles and turn off and make another attempt. We came to a set of traffic lights on red on the 4 lane highway. We were in the third lane. A police car cruised up in the fourth lane next to us. There were literally thousands of cars waiting behind us in 4 lanes for the lights and the same number waiting for the lights in the opposite direction. It was a crossroads.
I innocently leaned out of my window and politely asked the cop in the passenger seat if he knew a way to get back to Logan as we had missed the ramp. He asked if we were English ! How could he have known !!!
He then got out of the car, walked across the highway, waited until the lights changed to green.....and then held up thousands of Bostonians who were trying to get home......and indicated to me to do a U turn....so we could be on the right side for Logan !!!!!!!
It was a wonderful and kindly thing to do and we will always remember it. Can you imagine a U.K. road traffic cop doing that !!!
I hope you don't think I have hi-jacked your post with my little story. I just thought it would show how driving in the States is fun.
May I finally ask who you are using to rent your car from. We have always used Hertz and they are generally pretty good. If you are a little concerned about your first time in an automatic, Hertz have a range of " prestige " cars that are mostly European models. The model of choice is guaranteed. So you could choose one of those and then take a test drive here in the U.K. with a dealership to see how you like it. Just a thought.