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
Add a hard drive to your laptop by replacing the optical drive with a second hard drive. Compatible with all Model A1283 Mac minis Mid 2010 Model A1347 Mac minis iMac Intel 20"" EMC 2266 iMac Intel 24"" EMC 2267 iMac Intel 21.5"" EMC 2308 and 2389 iMac In — 50+ available at 39.95 each.
Transform any bare hard drive (SATA or IDE) into an external storage solution. Compatible with all G3 Lombard and Pismo PowerBooks, all G4 PowerBooks, all iBooks, all Mac Minis, all G5 and Intel iMacs, all MacBooks, all MacBook Pros (excluding MacBook Pro — 50+ available at 29.95 each.
Yes you can depending on the model number. I managed to replace my existing factory fitted HD with a SanDisk 256GB Extreme II drive on my iMac 27" 2374 model. Replacing the hard drive removes the internal HD thermal sensor which is needed by the iMac and a lack of this sensor will cause the fan speeds to constantly run at maximum.
To prevent the iMac spinning its fan's constantly at full speed some people have used software to override this and there is another workaround! If you have a spare thermal 2pin sensor such as one off an older iMac's optical drive unit (or one purchased online) then you can use this. Carefully plug this into the HD thermal sensor plug (careful not to damage the connector socket) and fix the replacement sensor onto your SSD with sticky tape.
I have done this procedure and confirms this works. The iMac is fooled!
My iMac is 10 times faster running an SSD! and this procedure allows you to maintain your optical drive unit.