I have never used Steam before, but now I want to install Steam because there are some games that are having a Cyber Monday sale today. The problem is that my Windows PC is broken at the moment, so I would have to install Steam on my Mac.
If I create a Steam account with my Mac, could I then later use the same account if I install Steam onto a Windows PC?
The same Steam account will work on all platforms. Most games don’t work on both Mac and Windows, though, so bear that in mind. You can find out the compatible platforms for each game on their Store page, at the bottom with system requirements. Steam should still always let you buy any game (available in your region) for any platform.