Download
this tax form, complete it, and send in the estimated payments. Box 1 is estimated AGI. If professional poker is your only income, estimate how much you are going to win for the year. Make sure you completed self-employment tax on line 11.
You should also check your state to see if you will owe taxes there, go to their web site, and get the equivalent estimated taxes form.
You are allowed to annualize your estimated payments, so you could calculate your estimated AGI based on Jan-Mar 2007, and make your April 15, 2007 estimated payment based on that. Then re-calculate your estimated AGI based on Jan-Jun and make your June 15, 2007 estimated payment based on that. If you run hot or cold in the 2nd quarter, your second estimated payment may be higher or lower.
Good luck,
-Tom