JRPass - 7 days

Hi!
I'm planning on buying a 7 day JRpass for my upcoming trip to Japan. I have several questions

1) I understood I have to activate it when i want to start using it. I can only do this at the airport or are there several locations where I can activate it?

2) Once I activate it I have to use it for the next 7 consecutive days?

3) I want to travel from Tokyo to Kyoto, Osaka, Nara. Is this pass enought?

Thanks!!