Well, you should go to the My Stuff tab to your left. Then go to the Messages tab. Read the text at the top of the page. There should be a link (Signified by blue text) saying, "If you're looking for an old notification you received, click here.
As you are new, and probably do not have many notifications the message asking you if you want to duel should be easy to find. Click on it, then you will have the choice to accept the duel or not.
Hope this helped. :)
