I only buy either hard drives or SSDs at OWC:
www.macsales.com
They are Mac centric and have quality products. Note: All hard drives will fail sooner or later. If you decide on a hard drive, stay away from 5400 rpm spinning drives because they are too slow; 7200 rpm is acceptable. If you choose an SSD, they are super fast, but much more expensive.
So, basically, you plug it in and go to Disk Utility (in your Applications folder). Highlight your external on left and erase it first. Most have some irrelevant software installed - it is not needed. When finished, format it Mac OS extended (or APFS) and then it is ready to go. If you want to clone your internal so you can boot from it, then get CarbonCopyC;oner or SuperDuper. Install that and then run it following their directions - you will have an exact clone copy of your current system on the external drive and can boot from it and run your normal tasks from that.