7 or 14 day pass?

Hi there!

Here are some answers for you :)

. As I understand I can activate it at any day I want during my stay??

Yes, that is correct

Can go the office and ask for specific day of activation I want, or it has to be this day when i go to the exchange office?

Yes, you can ask for a specific day any day within the following month.

Then i plan to pay extra, not during the jrpass, for transfer from narita to tokyo, return trip to yokohama, and return to narita? Do you think it is good idea or it is better to buy 14 day pass?

I think your plan of using a 7 day pass is best here. While you can use the JR Pass within Tokyo and for the travel to and from Narita, I think you'll get better use from either the [Narita Express + Suica deal] or the [Keisei Skyliner + Metro Pass deals] that are available at Narita. These will get you into town and give you some free travel around Tokyo also.

My last question is if hyperdia.com is good site to check train timetables?

Yes! Hyperdia is a great resource :) You can also use our Japan Rail Pass map to help plan your trip.

Hope this helps!