Really only a few options:
Size, Cost, and Type.
Size is pretty obvious.
Cost is pretty obvious, too.
Type is HDD or SSD. SDD is flash memory, while HDD are normal spinning hard drives. If you don't know anything more about them, they are faster to both read and write but are more expensive. If your processor / graphical process is top notch, then your bottleneck in processing might be in the read/write speed, beyond that I would say just get a few HDDs.
Note: It is good practice to save your raw files on one HDD, then save the exported file onto a different one so you aren't reading and writing from the same hard drive at the same time.
I should also say brand really doesn't matter. Who you are buying it from provides the warranty.