What pre-paid card should i use?

I am using it to take railways during my short trip (8 days) in osaka and tokyo. Just for your reference, i am 17 and i am a foreigner. Thank you.

If you are doing a roundtrip between Tokyo and Osaka then you're probably best off getting a nationwide JR Pass. Otherwise, get an IC card (Suica/Pasmo or Icoca, whichever is the first one you come across) for local travel and pay for regular shinkansen tickets separately.

I agree with yllwsmrf. You may find it advantageous to use the Japan Rail Pass, especially if you are doing a round-trip between Osaka and Tokyo.

You can check here, on Hyperdia, to see if the pass pays off for your travels. http://www.hyperdia.com/en/

A benefit if you use the pass is that it is valid on all Japan Rail trains (and their buses and ferries). So when you are in Tokyo and Osaka you will also be able to ride Japan Rail trains with your pass and will not have to pay extra. But, keep in mind that the Japan Rail Pass is not valid on trains belonging to another company (such as Toei or Tokyo Metro in Tokyo).

Sometimes there is confusion when people read Japan Rail Pass and think of it as Japan "Rail Pass" (ie. a rail pass for all trains in Japan), when really it is "Japan Rail" Pass (ie. a pass for trains belonging to the Japan Rail company).