Here is an example of the mounting plate you may need to buy (this is just an example there are dozens to choose from).
Amazon.co.uk User Recommendation
As well as the SATA cable I mentioned above you will also need a power cable to connect the SSD to a power source, though most PCs come with spare power connectors inside the case so you wont need to buy one.
Note desktop PC hard disks are called "3.5 inch" and laptop hard disks are called "2.5 inch".
SSDs are also 2.5 inch so this is why you need to 3.5 inch to 2.5 inch converter so the SSD fits in a 3.5 inch slot.