You should learn a bit about the product and select one according to your preferences.
Minimum facts you should learn about the an external hard drive.
Connecton: (USB 2.0), USB 3.0, Firewire...
Capacity: 500GB, 750GB, (1TB), 2TB ...
Brand: Hitachi, (Seagate), Toshiba, Western Digital ...
Drive Size: 2.5 inch, (3.5 inch)
My recommendations in parenthesis, reasons:
Seagate (which bought Maxtor) is my opinionated brand of choice for dependability. USB 2.0 is pretty much a standard whereas USB 3.0 is newer (not supported by older systems and more expensive) and firewire isn't very popular. Capacity is up to you, but 500GB or 1TB are the ones typically on sale. 3.5 inch because you want a back-up solution and not a more portable (2.5") solution.
Based on my recommendations, one could be had for around $50. Look for DEALS and not just "sales".