What should I put in the box that says "beginning balance" in the home mortgage interest worksheet?

[JulieCo]

It is the balance for January 1, 2018.  If you received a 1098 form, the January 1, 2018 balance should have been provided for you. 

If you are being asked for a balance from the beginning of 2019, then you will need to find that on a mortgage statement or to call your lending company to get a balance.

These balance amounts are being asked for due to change in tax law related to the mortgage interest deduction.  There are now different mortgage interest deduction limits based upon when the mortgage was originated and what it was used for, so portions of a mortgage listed may include interest that is not deductible.  Therefore, the ending mortgage balance is needed to determine the deductible portion of the interest being entered.  You can find your ending mortgage balance information on your statements or by checking with your mortgage company.

Here is more information about the tax law and the provisions for now being able to deduct mortgage interest:

https://ttlc.intuit.com/replies/3300473