How much to pay with Seishun18 Kippu?

using http://grace.hyperdia.com/cgi-english/hyperWeb.cgi as reference, journey from
shinyokohama to Shinosaka via Nozomi, basic fare=8190yen, charge for reserved seat=5740 yen.
if i use Seishun 18 Kippu ticket, is it correct i can use the kippu ticket and pay only extra 5740yen for the reserved seat?

You will have to pay the full fare. The Seishun 18 Kippu is only valid on local and rapid trains!

More details:
http://www.japan-guide.com/e/e2362.html

Does [JR Special Rapid Service] Line and [JR Nara] Line in Kyoto covers by Seishun18 Kippu Ticket?

c0met,

Does [JR Special Rapid Service] Line and [JR Nara] Line in Kyoto covers by Seishun18 Kippu Ticket?

As explained on the page Uji provide a link to above, the Seishin 18 ticket is valid on JR "Local" and "Rapid" trains, including "Special rapid" trains, so yes, it is valid on those services.

If you are unsure where your Seishun 18 Kippu is valid, uncheck all the checkboxes when searching routes with Hyperdia and it will only show valid routes.

Not exactly because it (search results) might give you some non JR trains as well. Only valid on "JR" ones.

Thanks for that piece of information, it had not occured to me, since I've had no trouble using Hyperdia to arrange my seishun travels in the past, but looking it over now I see that you're correct. I'll have to be more careful in the future.

It says we need to "reservered" seats for nozomi, how to reservered a seat?
how long ahead must we reserved a seat?
Is it normally Full?

It says we need to "reservered" seats for nozomi,

Seat reservations are not required! There are three cars with unreserved seats.

how to reservered a seat?

At the ticket counter of JR railway stations nationwide.

how long ahead must we reserved a seat?

Reservations can be made from one month in advance up to departure time.

Is it normally Full?

No, it is normally not full. But it is not very uncommon that during some busy times, some trains are fully booked.

How can we purchase the Seishun18 Kippu ticket? at stations' vending machines or at JR ticket counters? We plan to purchase it at either Ueno Station or Shinjuku Station. Do you have location map for the ticket counters? Thanks.

You can purchase them from a manned ticket counter. I'm not sure about automated ticket machines though. Here are maps of major JR stations:

http://www.jreast.co.jp/e/stations/index.html#S