Hi Cherrie,
The main thing to bear in mind when you're planning your trip is that a 14 day JR Pass costs ¥45,100 and a 7 day pass costs ¥28,300.
If you think you will spend more than that within 7 or 14 days, or would like the flexibility and peace of mind to put a cap on your travel costs, then a JR Pass is a good idea. In general for example, a return Osaka to Tokyo covers the cost of a 7 day JR Pass, making all further JR travel for the 7 day period effectively free.
The JR Pass will cover travel to and from the Osaka Kansai and Tokyo Narita Tokyo airports, and covers all JR trains except for Nozomi and Mizuho class bullet trains (you can ride all the other types of bullet train/shinkansen, and can reach all parts of Japan).
There is a good JR overground rail service in both Tokyo and Osaka - here's a map of JR lines in Tokyo and a map of rail lines in Osaka, including JR. It's important to remember that the metro services in both these cities are not operated by JR so you'll need to purchase tickets for this separately if you want to use it.
You can look up precise train timetables and costs at the Hyperdia travel planning website and learn how to use Hyperdia for JR Pass holders through our video blog to help you cost things out while you plan.
Hope this helps!