First have a look at the manual or help file of After Effects to get acquainted with that program.
Second, read the manual/help file thoroughly on topics you want to know more about.
That is where the RTFM comes from.
And here is a link to official Adobe support and help documentation.
http://www.adobe.com/support/aftereffects/
And finally, I can't answer your question precisely, as I haven't touched AE in years. But in PS one can change the background of a new image during the "New Image" dialog.
PS: Now I can answer your question.
Choose Composition > Background Color, and then click the color swatch or use the eyedropper to pick a color.
from
http://livedocs.adobe.com/en_US/Aft...WS3878526689cb91655866c1103906c6dea-7f07.html
via
http://www.google.com/search?hl=en&...=after+effects+composition+background&spell=1
Ergo you spend more time here waiting instead of looking for yourself.