Because you did not pay taxes on the money when it was contributed (for a standard pre-tax 401k), everything is taxable when you withdraw it, both contributions and earnings. If you are under age 59-1/2, you will also be subject to an additional 10% penalty for early withdrawal unless you qualify for one of the exceptions.
Note that the broker is not required to mail the 1099-R until January 31, so you might not receive it until early February. It might also be available through your online account.
