Every purchase of Destiny: The Taken King includes one level boost. This comes in the form of a special item called the Spark of Light.
To acquire the Spark of Light, create a character or hop on an already existing one and visit the Postmaster (seen in the image above) at the game's main city hub, Tower. The Postmaster is one of the main NPCs in this area, and can be reached not far from where you spawn to your left. Speaking to the Postmaster will open up a menu where you can find the Spark of Light in the Consumable section. It has no cost associated with it.
Upon using the Spark of Light, your character will immediately become level 25 and be granted a full set of armor, weapons, and several useful items. You are now ready to get started with The Taken King's content, which spans level 25 to 40. Good luck!