You have options. If you don't want to go the easy route and get an add-in card, you can do a clone and replace.
Depending on what size your SSD is and your intended use... I am going to assume things here.
- Disconnect the non-OS drive.
- Plug in the new SSD
- Clone the HDD to the SSD, moving your OS there.
- Disconnect the HDD
- Reconnect the second HDD
- Restart your system and in your BIOS make sure you designate the SSD as your boot device
- If you're SSD is large enough, you can copy over files from your second HDD
Granted, I assume you will stick with two devices. The add-in card is a cheap option to have more drives unless you replace your motherboard. If you get an add-in card, be sure you have SATA III cables that will reach, odds are you need to buy new ones.