The most commonly used operating systems include Windows, Mac or Linux. All three systems have their good and bad points, but it's important for you to choose a side before you start, because it affects your software choices and possibly your hardware decisions too. This is definitely the case with macOS (formerly called OS X), which only runs on Apple's Mac family.
Here's a look at the pros and cons of each:
Windows software has the lion's share of the market, with the widest range of programs available. Though Windows 7 is still popular on older PCs, new Windows computers will come with Windows 10, which is designed to work across a wide range of devices including tablets and which comes with touchscreen support built-in.
MacOS is designed to work specifically with Apple hardware, providing tight integration that offers advantages in ease of use and consistency across programs. Many programs for Windows have macOS versions and many macOS-only programs offer file-format compatibility with Windows programs. You can install Windows on Macs using macOS's built-in installer utility called Boot Camp. This will let you run Windows natively on the Mac hardware without any software emulation, to give you full performance. You can also run Windows OS and programs using virtualisation software such as Parallels Desktop for Mac, VMware's Fusion or Oracle's Virtual Box. These programs let you install other operating systems such as Linux. Whether you use Boot Camp or a virtualisation program, you'll need to purchase the Windows operating system separately.
Linux is generally free, as are most Linux programs, and it can run on a wide range of PCs as an alternative to Windows. There are many flavours of Linux, with the most popular being Ubuntu.