The lens you choose is completely dependent on what you want to shoot as different focal lengths create different effects. However as you’re probably just starting out, it’s likely you don’t know where your heart will eventually take you in this regard.
Therefore I always suggest a 50mm lens for a couple of reasons.
Firstly, 50mm is a versatile length that can be used for a few different genres of photography. Secondly, both Nikon and Canon sell an entry level 50mm lens that is super cheap but excellent value for money, and will out perform any kit lens that you may have bought with your camera.
http://clicklovegrow.com/how-100-changed-my-photography-forever/
If you already have an idea of where you’d like to take your photography, research a little further, starting with these articles.
http://clicklovegrow.com/6-things-you-need-to-know-before-buying-a-new-lens/
http://clicklovegrow.com/what-lens-should-i-use/