Go to Whiterun and talk to Jarl of Dragonreach to receive a quest. Finish this quest, then go to the Companions faction to get a series of quests to do. Once you finish these quests, they will give you one final task; they want you to gather some items and then meet them in the Underforge. It is here that you'll enter into the ritual to become a Werewolf. There are some pros and cons to being a werewolf. Your health and stamina increase by 100, your sprinting is faster, you gain a new shout that scares enemies and summon wolves, and your hand-to-hand combat receives a melee damage bonus. The downsides to being one is that you can't use spells, equip items or loot corpses while in wolf form. You're also not able to talk to NPCs and guards will hunt you down if they see you in wolf form. Some NPCs will be scared of you or even try to attack you and if you're caught transforming into a werewolf, you are convicted of a major crime. The last downside is that you're weak against silver. Transforming into a werewolf is not permanent, as it's a power that only lasts around a few seconds at first. To get it to last longer, feed on defeated enemies.