On the flight - you'll be fine, just pay attention to all the announcements airside and on the plane and ask the crew if there is anything you don't understand, they are there to help.
On transatlantic flight of that length you will probably get two meals - the type and amount will depend when you leave. So for example last time we went to NYC we left London mid morning so we got lunch not long after take off and then a hot drink and biscuits about an hour before landing. The food is never amazing but i always eat it - it's something to do for an hour or so!
Don't worry too much about a window seat - a lot of the UK to NYC flight is over the sea so it is a bit dull for a lot of the time!
If you are going over night be considerate of your fellow passengers, a lot of people use that route a lot and taking an over night flight is common (and the NYC to UK leg destroys you)
Things to take with you in the cabin - good headphones - there will be free entertainment on the screen in front of you but the free headphones are horrid. I also take chewing gum, a bottle of water (bought after security), tissues, ear plugs, a scarf/blanket, (the cabin can be cold) handcream, some deodorant/body spray for when we land ) and a book. Take any medication you may need over 7 hours. Wear comfortable clothes, there will be a lot of sitting.
Before you go you need to apply for an ESTA - this is a visa waiver, most people are eligible but if you are rejected you have to apply for a Visa which takes ages, so apply in plenty of time.
Once you are in New York - as people have said prepare of a long queue at immigration, it is just a thing. Be careful with any food you may have taken with you, there are pretty strict rules on that.
There are a few ways to get form JFK - if you can get a cab the amoutn the can charge is set - this is expensive but probably a good option if you are nervous or are not familiar with public transport. Otherwise there are trains and subways to get you to the centre of the city, The journey is long but cheap.
New York is BIG, it is easy to walk in but a lot of places are too far to walk - the best way get around is the subway. It is crowded and a bit grubby and old fashioned, but fine. You can now use contactless cards to pay for it (you tap on barriers like in London for the tube).
I would take something like a monzo or revolut card but also some dollars, America is getting on board with contactless payment but cash is still useful.
As others have said budge tin a 20% tip for all food and drinks.
New York is A LOT it is amazing but huge and very busy. Its like London on steroids. But it is also an amazing place so have fun.