From the Home page of your Service Planning account, you will see any assignments waiting for your response.

You can Accept, Decline, or mark your availability as Tentative. If you have already selected your response via the email notifying you of your assignment scheduling, you will not have to do this again. This also applies to your text response if you have that set up within your profile settings (to turn this on: click your name My Profile Communication Settings) 

If you have marked yourself as Tentative, you will still need to let your Leader know via response whether or not you can serve once you are certain that you can/can not fulfill your assignment request. You may also change your response on an assignment from here as well.