How to get a specific JR Pass in Japan?

Martin is again wrong on getting your money's worth - going Tokyo-Kyoto-Tokyo doesn't quite cover the cost of the pass. Almost, but not quite. Taking the Narita Express would however put you over the top, but more than that, there are numerous good side trips you can take to see more of the country and increase your savings. Near Tokyo there is Nikko, Karuizawa, Izu, Kusatsu, Kamakura/Enoshima, etc.

The OP's question "have a pass to conclude all these rides within 7 days" makes no sense. It's entirely up to you to get on and get off the train within 7 days. A few days in Kyoto is not enough to see all that is there - not even a few weeks in Kyoto would do all that. Nara is another place you should definitely not miss.
https://www.youtube.com/playlist?list=PL7LcPcmoSg-jGyApXWSTIKwkioxuAKwt5

You must get the exchange order before arriving in Japan though. It is IMPOSSIBLE to buy the full JR Pass after arriving in Japan. Any big travel agent that sells tickets to Japan can sell the pass. Better to buy from a well established place and not some fly-by-night outfit you've never heard of.

There is however a better deal. If you only want 7 days, and won't be going beyond or stopping in between, then there are these 2 cheaper deals.
http://www.japanican.com.hk/en/tour/detail/VJOPENTK1/
http://www.japanican.com.hk/en/tour/detail/VJOPENTO1/
You'd need to pay for travel within Kansai, but you can still come out ahead.
And no, you cannot ride the Nozomi trains with the full JR Pass - but most others are no problem like the Sakura. It does add another 20-30 minutes, but not a huge deal. With the japanican tickets you can in fact ride the Nozomi, but there is no advance seat selection.

JR Pass covers ALL JR train lines (except a few trains) in Japan. So with one JR Pass, you can land in Tokyo, go to Kyoto, and stay there or can come back to Tokyo.

The nationwide JR pass only pays off if you do a long distance return trip within the time frame (i.e. Tokyo - Kyoto and back)

You have to buy the JR pass voucher in your own country and exchange it within Japan.

You can then take the JR NEX from Narita airport to Shinjuku, use it on all JR lines in Tokyo (but not on subways or private lines) and then take the shinkansen from either Tokyo station or Shinagawa station to Kyoto and back.

Just go for it, it's worth it. You cannot lose unless you go nowhere. Yeah, every time you get on the Narita Express that's about 3500 yen. The JR offices at NRT are open very long hours! Go to the JR Rail Pass office at Tokyo Station and they will brief you on each and everything a JR Rail Pass entitles you to. It's not quite a mission from God, but pretty close to it.

You do not purchase a JR Pass inside Japan. You can only purchase it (usually from a travel agent) outside of Japan where you are given a voucher upon payment. When you arrive in Japan, you give the voucher to a JR rep at one of their kiosks located in Narita airport and you are then given the JR Pass. You will stipulated when you want the 7 day pass to be effective (when the 7 days begin) which in your case would be immediate.

A JR Pass is only good on transportation networks of trains, buses, ferries, etc. that are owned and operated by Japan Rail (JR). Not all trains, buses, etc. in Japan are owned and operated by JR. Japan Rail was formed after Japan National Rail (JNR which was a government owned company) was privatized.

The 7 days JR pass can be used for your itinerary - you just have to make sure to find the line owned by JR for the destination you want.

Thanks for your words!

Can I get the JR Pass when I arrive Narita?

I checked my Hyperdia and showed to take Nozomi from Shinagawa to Kyoto, but Nozomi doesn't seem being covered by JR Pass, is that correct? If so, how to ride around it?

The JR Pass is about US$235, I just want to save a few bucks here and there (budget travel) lol!