Relax, and check out your mini-menu for monitors over there in the menu bar. See "Detect Displays?" That's your buddy, right there -- it will tell your Mac to recheck the connected monitors and adjust resolutions as needed. Note the model number of the external display shown; if everything's connected as it should be, your Mac should autosense the type and capabilities of the display without any intervention.
For an even quicker trigger on display detection, try Cmd-F2 (brightness up) on your built-in laptop keyboard. Cmd-F1 will toggle display mirroring, also handy in a pinch.
There are several ways other ways to get external displays to behave; the oldest, for laptop users, is to sleep the machine and wake it back up -- this usually triggers a display detection when all else fails. There's also a Detect Displays button in the Displays preference pane.
Also, to answer a reader question from the comments: it's easy to specify which display acts as the primary monitor in a dual-display setup: just drag the teensy proxy menubar from one display to the other, and your menus, drive icons etc. will follow.