Simple way to back up my Windows PC before I start messing with servers?

I'm about to start trying Linux stuff and I'd like to back up my files first, in case I wipe something by accident. My main computer is a Windows PC and I mostly use it for work stuff and photos. I was thinking of just copying folders to an external USB drive, but I'm not sure that counts as a real backup.

What should I actually keep in a backup? Just Documents and Pictures, or things like browser bookmarks and saved settings too? Also, is there a free program on Windows that does this on a schedule, or is copying by hand fine for now? Sorry if this is basic.

This really depends on what you are looking to restore. Are you looking to be able to restore it to 100% of what you have now (e.g. do you want all of your applications and everything)? If you're just worried about files I would just back up your user folder.