Should I buy a JR Pass? Which one?

Hi, everyone, I am touring Japan in week's time. I'll be arriving at Narita airport, and hotel will be at Shinjuku. After two days in Tokyo, I plan to take a train from Tokyo to Kyoto, possibly also to Osaka, even to Nara, over two days. I wonder if I an buy (maybe a 7 day?) JR pass that allows me to take the N'EX train (or some other train) from Narita to Shinjuku, and then after 2-3 days, from Tokyo to Kyoto-Osaka-Nara, then back to Tokyo, all using the same JR Pass? On Klook.com, the JR West pass (for Osaka/Kyoto area) does not start from (or include) Tokyo, so which JR Pass will cover Tokyo to Osaka/Kyoto/Nara areas? Will that same pass also cover Narita Airport to Shinjuku of Tokyo?

How much is it and is it more worthy than to buy separate tickets? A link to a webpage showing such a Pass would be greatly appreciated. I also wonder if I need to book the train ticket (or a pass) beforehand or just to buy it right before boarding? Should I reserve a seat? It's all quite confusing.

Also within Tokyo, what is the best public transportation ticket to have for tourists? I guess I will be using the subway a lot (buses can be confusing). Maybe 72 hour Tokyo subway ticket (maximum duration available on a ticket)?

Thank you all!