This tutorial video explains how to install a SSD into a 2012 Mac Pro.
You will require an adapter due to the fact SSD’s are 2.5 inch and standard HDD’s are 3.5 inch. I used an OWC Drive Sled (http://eshop.macsales.com/item/Other+World+Computing/MMP35T25/). It is a custom sled designed to align the SATA ports of the SSD with the motherboard.
The SSD I installed was a Samsung 840 Pro (http://www.samsung.com/au/consumer/pc-peripherals/solid-state-drive/solid-state-drive/MZ-7PD256BW).
You will require a phillips head screw driver to install the SSD onto the adapter. The screws should be supplied with the adapter. A 3.5 to 2.5 inch adapter would also work.