Mario Kart 8!

[Evastio]

I'm Skyblade12, but I have no idea how to add or challenge people on the Wii U.

I sent you a friend request on Miiverse. Simply friend people on Miiverse if you want to race online (you'll need to search for them online with the "search members" option), select friends when you choose online in Mario Kart 8 on the main menu, and you'll be able to join any of your friends in a race if they're online (they'll have a checkered flag beside their name if they're online in Mario Kart 8).