There's a ticket booth just east of the sidewalks that lead up to the base of the monument. You can't miss it.

Go to the NPS site for the Washington Monument (http://www.nps.gov/wamo), where you'll find links for online reservations via Recreation.gov. Show up on the day and you might get lucky, though there's no guarantee you'll get the times you want/need. Best to book ahead of time, obviously. Note there's a small administrative and postage fee for booking tickets online.