Thomas -

Heres the guide to gain access to the HD: iFixIt HD Guide

If you are not using your optical drive, consider replacing it with the SSD instead. Here's the guide to take the optical drive out: iFixIt Optical drive guide.

In either case you'll need a 3.5 to 2.5 bracket (depending on which way you go you'll need different bracket types) and in the case of the optical drive the addition of a optical to HD enclosure Optical to HD SATA enclosure

I would strongly recommend getting a Firewire (if you can still find one, it's getting hard) or USB HD enclosure to house your old HD as a backup drive and using it to prep your SSD. Or use this kit: Universal Drive Adaptor to prep the SSD before you install it.

And lastly, the SSD you get must be able to run at SATA II specs