Please read before you post, young Wizard. There's a post at the very top of the Grizzleheim and Wintertusk forums that explains in detail. You must complete Grizzleheim before you can enter Wintertusk.
When you are Level 25 you will be able to begin the Grizzleheim quest Lending a Paw.
When you are Level 30 you will be able to begin the Grizzleheim quest Full Grizzle Jacket.
When you are Level 40 you will be able to begin the Grizzleheim quest The King and I.
You will begin your journey in Wintertusk after you have completed the "Blood Brother" quest in Grizzleheim.
If you have completed this quest, speak to Merle Ambrose who will give you the quest "Cold News".
If he does not have the quest 'Cold News' for you, have haven't completed the "Blood Brother" quest.
When you begin your quest to rescue Grandmother Raven, you will be sent to Hrundle Fjord, the main area in Wintertusk.