How do you become a werewolf in the elder scrollsIV

if you're talking about oblivion, you cant. There is no way in oblivion you can become a werewolf, although some people believe you can become a werewolf in the shivering isles. But as far as i know you cant become a werewolf, or of course you could do a MOD, which is some thing that allows you to do anything you want, thats why on youtube you see these people doing crazy things like getting a pegasus or a dragon for an example.
If you are on about Skyrim, follow these instructions:

1. Complete a quest for the Jarl of Dragonreach in Whiterun.
2. Finish every quest they give you until you complete the chain, and the Companions will send you on one final job.
3. In the Underforge, you'll enter into the ritual to become a werewolf. Now, you'll need to figure out where to find a cure.

Done.