If your light has a 12v plug on it, then most likely it was designed for interior use only, and it won't be waterproofed. So the first time it gets wet, it's done.
Like was just said, you'll need a switch between it and the battery, and you should also wire in a fuse, as close to the battery as possible. Try to find a waterproof switch also, with a regular switch you'll probably end up with it ruined after a few rides because they're not meant to be exposed to moisture, dirt etc.
But if that's not a waterproof light, then don't even bother wiring it up. If it is waterproof, and you do install it, make sure you mount it where it's not going to get broken easily, and mount it securely enough that it won't vibrate around at all. Definitely install a fuse, mount a switch in your dash panel, and use some decent automotive wire, not speaker wire because the thicker the insulation, the better. If possible, solder and shrink wrap the connections. If not, at a minimum use some insulated crimp-on terminals. If you're new to electrical stuff and wiring, I could probably help you out with a simple wiring diagram.
Trust me, I know my neons ;D