First check out this thread (and search the forum for others), it's full of ideas for neatly mounting GPS units using the standard accessory socket.
Garmin GPS and Center Console
If you're set on hardwiring your Garmin, I think you would need something like this ...
Amazon.com: Garmin Mounting Cradle: Electronics
You would also need to decide where you want your Garmin ... whether you're going to use a friction mount, suction cup mount, air vent mount, or something fancier like a ProClip mount ...
ProClip Mounting System*-*Vehicle Mounts
Also you'll need to figure out how you want the power cable exposed ... whether you want it dangling loose, or drill out a discreet hole somewhere in your dashboard for it to stick out of. This is probably too ambitious for a first project, so you might want to check with a local car stereo / alarm shop and see if they can do the work. Hope this helps, or at least gives you some ideas for next steps.