You will need at least two pieces of hardware: a 2.5" sata SSD and an optical disc caddy which will be used to install the hard disk drive into the optical disc slot.
Personally, given the age of the Macbook, I would get the cheapest 120 or 256 GB SSD you can find like a Samsung 850 EVO or Crucial MX300. For the optical caddy, you can just buy one from Amazon and eBay.