The next world Jamboree will be in 2019 in North America, and is being organised by Mexico, Canada, and USA. Have a look on Facebook/2019wsj and twitter @wsj2019 or #wsj2019
The site is in West Virginia and is already in use as a scout campsite. The Summit is a high adventure base. It should be an excellent site for an exciting Jamboree.
So you want to go. Who can go? You will need to be between 14 and 18 for the Jamboree to attend as a participant. That means you will be between 10 and 14 now in 2015. You will need to apply through your local scout association. In the UK that is likely to be a district or county selection panel which will pick from all the local applicants.
If you are older you may be able to go as a unit leader, or a member of a support team. These positions are also on a selection basis. If you are younger, you may be able to attend as a day visitor, but it might be better to wait until the next time, which could be in Korea or Poland or somewhere else.