Most any external drive that you can buy may be used with both a Mac and a PC.
Most are already formatted for PC use, so should also work with the Mac.
Out of the box, the Mac can use drives that are formatted for PCs using FAT32 and ExFAT.
If you happen to buy one that is formatted for Mac use, then you'll want to reformat it to one of the PC formats above.
Some drive brands that are popular are LaCie, Toshiba, Western Digital, Seagate, SanDisk and Sony.
Many veteran users here will recommend external drives from macsales/OWC.
Laptop class 2.5" drives are popular because they are lightweight and portable. They get their power from the direct connection to the computer.
Desktop class 3.5" drives are bulkier and will have their own ac power adapter. They're less portable, but are often more robust.
SSD drives are fast. HDDs - not so much.
HDDs usually cost less per GB of storage. SSDs are a bit more expensive, but have become more competitive.
Do a search on Amazon to see a wide range of drives of all types, and if you have more specific questions about particular drive, please post again. 🙂