Paying taxes on IRA to Roth conversion when tax was not withheld

I just had my finance advisor transfer $200,000 from my IRA to my Roth.  He did not perform any tax withholding on the transaction and I did not pay any estimated tax for the year.  I retired at the end of last year and did not have any income until this IRA to Roth transfer.  Do I need to pay tax right away and how do I do that?