I mean if I need to create a new character or if I can access the DLC content with a previous character (a character I had before buying Ashes of Ariandel).
You should not have to create a whole new character to access the DLC.
Here is how you access it:
...proceed to speak to an NPC at the Cathedral of the Deep. Warp to the Cleansing Church Bonfire kneeling beside the altar between the church's large doors. Speak to him and exhaust his dialogue to trigger a cutscene and you are given a piece of the painting at which point you will be transported to the beginning of the DLC.
The Ringed City DLC is also accessible on existing characters after having completed the game's main story line or finishing Ashes of Ariandel.
The Ringed City DLC is accessed by traveling to Kiln of the First Flame, and finding a new bonfire before Soul of Cinder that transports you to the DLC location. There is also a bonfire you can find in the Painted World of Ariandel, in the chapel by Sister Friede.