C is one of many programming languages that are useful to learn as a beginner. MagPi, the official Raspberry Pi magazine, put together a book that helpfully walks you through the basics. As you'd expect from MagPi, the guide leans on learning C on a Raspberry Pi, but it's not necessary to use the microcomputer to follow along with this book as the lessons you learn are applicable on any operating system. The miniature book works as a good introduction to a number of concepts, walking your through everything from variables to reading and writing files. It's a good place to start to learn if you're even interested in C to begin with before you move on to something bigger. You can snag the PDF for free or download it for a few bucks inside the MagPi app.
Learn to Code with C [MagPi]