This is no big deal, it's even covered in warranty if you don't damage anything.
- Get your hands on something called an OptiBay. It's an enclosure for 2.5" hard drives that fits into the MacBook's SuperDrive slot. Those can be bought on eBay <100$ or straight from MCE. Make sure you get the right model for your Macbook Pro.
- Buy an appropriate 2.5" SSD, I think that any will do fine, whatever fits your budget.
- Go to iFixit and find the specific guide for opening your Macbook model. They have a lot of high quality photos and even give you a list of the tools you need for opening the enclosure and accessing the SuperDrive.
- Pop in the SSD drive into the OptiBay, replace the SuperDrive with the OptiBay and boot up the Mac
- You should now be able to format the second drive using Disk Utility.
- Enjoy your additional storage!
Note #1: You don't specifically need an OptiBay, you could just put in the drive, but you will need to make sure it fits tight and stays where it is. This is especially important for "normal" hard drives. SSDs won't take damage if they rumble around in your Macbook, but you don't really want that, do you?
Note #2: I personally would recommend you to replace the primary OS X disk with the SSD and use a traditional hard drive as the second disk. That way your applications will run from the faster disk and stuff like large files and archives can be stored on the slower drive. You could do a Time Machine backup of your system before and then just restore it onto the SSD.