You can put your Library on any directly-mounted storage drive. To move it to an external drive, use Finder and move the file, then double-click it to open it.
As Kirby said.
Only make sure, the drive is formatted for mac - it needs to be formatted MacOS X Extended (Journaled). Aperture will not open a library, if it is on a drive with a different format. (see: Use locally mounted Mac OS X Extended volumes for your Aperture library). You can reformat the drive with Disk Utility, if it should be formatted differently. Reformatting will erase the contents, so you need to backup any previous content.
I'd recommend to repair the library using the Aperture library First Aid Tools, before moving the Aperture Library (see: Aperture 3 User Manual: Repairing and Rebuilding Your Aperture Library). If the library should have broken links, they are easier repaired on the original volume.
And don't forget tu update your backup, before relocating your Aperture library.
Is your library referenced or managed? Moving a referenced library would require an addiional step to move the originals as well.
-- Léonie