Under the "Composition" menu, go to the "Composition Settings" option.
There you can extend the duration.
No question is stupid. Well, certainly not this one
Besides Steve's suggestion (on target as usual), you can also drag your footage item to the Comp button in the project panel, and that will automatically create a Comp that contains that footage item and automatically matches its' duration, frame rate and dimensions. It's quite handy, but only when you want to create a new Comp. For an existing one, use the Comp Settings dialog as explained by Steve.
You can even use the shortcut ctrl-k (cmd-k if on mac) to open your composition properties.