What passes should I get?

Hi there,

Yes the JR Pass works for this route and is also the only pass that covers this route. So I do think that this would be the best option (given that you also use it to make the return to Tokyo).

Hope this helps,
Daniel

And I also have one question, when we are going to Osaka do we need reserved seat? Or how is that work?

Hi again!

It is recommended but not mandatory.

Also see:
http://www.jrpass.com/blogs/reserving-seats-boarding-the-bullet-train