The calculator mentioned in the other solution states that for level 15, you'll need 62,125xp.
As mentioned in my other solution, I believe the fastest way to get this is in public (or private with a second/third controller) Horde mode, or versus mose matches to break up any monotony. I've yet to see anything in the game that boosts your xp total so quickly. Also, make sure you're working towards medals, as they each provide a significant boost.
Once this has popped, rejoice in the fact you're approximately 1.61% of the way to level 100! Yay!