shinkansen reservation question

On January 19th, which is a saturday, i want to go from tokyo to hiroshima via osaka. I would like to know, do i need to make the shinkansen reservations seperate.Do i reserve for the shinkansen from tokoy to osaka, and then ask for a reservation on the shinkansen from osaka to hiroshima? or should i just ask for a reservation from tokyo to hiroshima?
also, how long in advance do i need to reserve the seats, since its a weekend and i want to leave early (around 6)?
I will be in Japan a week before, should i go and reserve the seats then?
Thanks

You can buy the shinkansen tickets from Tokyo to Osaka ( the name of the shinkansen station is ShinOsaka ) and from Osaka to Hiroshima as soon as you are in Tokyo or up to maybe 30 min prior your departure. You ask for a reserved seat at the same moment ( slightly more expansive ) or non reserved ticket. With the non reserved ticket you can use any of the shinkansen on that day and you just look for a free seat in the non reserved cars. With the reserved seat you have a fixed train and a seat for sure on that train.
When you are in Japan as a tourist and you make the Tokyo - Hiroshima - Tokyo journey within 7 days, the JR Rail Pass is cheaper than single tickets bought in Japan.
( Yen 28 300 for the Pass compared to around Yen 38 000 )

Judging from your wording, you seem to have a Japan Rail Pass in mind in the first place, otherwise nearly everyone consider to travel directly to Hiroshima from Tokyo on a Nozomi. If that's the case, the earliest Hikari you could catch, available on a JR pass, leaves Tokyo at 6:36am. The connection: Tokyo 6:36--Hikari361--10:46 Okayama 10:51--Hikari457--11:33am Hiroshima
At the moment, both trains are still vacant according to the cyberstation site. You can book seats when or after you have your JR pass validated. You can book seats on both trains at one time at a ticket counter. I cannot tell whether or not you can secure seats on these particular trains on a short notice, but I guess it's quite unlikely they will be booked up on that weekend. The Hikari 457 may be busier than the other one, but even if it's full, you have only to keep standing for 40 minutes. No big deal at all.
Have a nice trip!

nearly everyone considers

May I know how to find out the seat availability on Shinkansen trains through the cyberstation? Do you have the URL? Is it in English? Thank you.

Cyberstation
http://www.jr.cyberstation.ne.jp/vacancy/Vacancy.html

You could search vacancies of reserved seats up to one month ahead. e.g. between 27th Dec and 27th Jan if you check today. I would see if your desired trains have vacancies on your behalf if asked. Please tell me what day, what time, what kind of train, e.g. Nozomi or Hikari etc, which segment, Tokyo to Kyoto? Then I'll tell you the results.

If your computer recognizes Japanese characters, and you'd like to check vacancies by yourself, especially for travels on Shinkansen trains, you might give it a go with my directory.

Boarding date: month / day
Boarding time: hour / minute
Type of train: when you click the arrow, there will appear seven lines with train names.
1st line; asking you to choose one of them
2nd line; Nozomi and Hikari
3rd line; Kodama
4th line; Hayate, Yamabiko, Nasuno, Tsubasa and Komachi
5th line; Toki, Tanigawa and Asama
6th line; Tsubame
7th line; Choose here if you aren't traveling on Shinkansen, e.g. limited express trains.

If you choose from between 2nd and 6th lines, station names will appear automatically in Japanese. They are all Shinkansen stations. Choose the original and destined stations, and press the blue icon. The result will come up.

Typical names of the stations tourists are likely to use:
東京 / Tokyo
新横浜 / Shin-Yokohama
名古屋 / Nagoya
京都 / Kyoto
新大阪 / Shin-Osaka
広島 / Hiroshima
Sorry if the chinese characters are garbled...
Google search will give you wikipedia pages with the name of the place in Japanese characters if you enter the name in alphabet. e.g. Sapporo

The site only works from 6:30am to 10:30pm in Japanese time.

Hope this helps.

Thank you for the detailed info and step by step guide to use the website. It really helps!

I have manged to arrived at the page that shows the status of train seats. Could any please help to translate the following legends for me? i only know ひかり = Hikari so far...Thank you in advance.

○：空席があります。 △：空席が残りわずかです。 ×：満席です。
−：該当する席はありません。または蘭�できません。
＊：喫煙席はありませんが、喫煙ルームが設置されています。

○：空席があります。--Vacant
△：空席が残りわずかです。--Nearly full
×：満席です。 --Full
−：該当する席はありません。または蘭�できません。 No service on that particular train (e.g. Green car seats), or unale to book.
＊：喫煙席はありませんが、喫煙ルームが設置されています。--No smoking allowed in all seats, but there are designated smoking areas on some of the carriages.

Thank you very much. :-)