Earthquake is one of the most straightforward, yet versatile, moves in any Pokemon game. It offers a base attack of 100, perfect accuracy without stat changes, and an unprecedented 10 shots apiece. Not only that, it's also effective against a huge range of pokemon, and is good against those who aren't even vulnerable to ground-type attacks. So how do you teach this fantastic move to your team in Pokemon Black and White?
Though it would undoubtedly be very handy in your first foray against the Elite Four, TM26 is, unfortunately, unavailable until you beat the game at least once. Doing this will open up new areas throughout Unova, one of which is Route 15. You can access Route 15 by travelling to either the Marvelous Bridge and crossing east or from White Forest / Black City and going west. The route is relatively short.
Once there, go down into the grassy section of the route and travel to the southeastern corner. You'll see a break in the grass, as well as a hole for a block and an item on the other side of the hole. Push the nearby block into the hole with Strength and run across to grab the item. This is TM26.