Q: How do I install windows on my Mac Air

There are different approaches, some use BootCamp which is free from Apple, https://support.apple.com/en-us/HT201468 while some will purchase Parallels and others use things like VMWare.  Best approach is what meets your needs the best.

BootCamp is straightforward and you can get help here on ASC since it is an Apple product.

In all cases you will have to buy Windows from MS.

If you want to use Windows exclusively or mostly BootCamp will let you boot to Windows or to OS X, but not both at the same time. Parallels and VMware Fusion let you run Windows as an app under OS X, so you get the best of both worlds. I'm partial to Fusion, because the virtual Windows files are compatible with all other VMware servers, so I can import a virtual machine from my office VMware ESXi server running over 500 Windows instances to my Mac and run it on the Mac, then copy it back later. Fusion also lets you run any version of Linux, and VMware has a library of pre-configured Linux installations that you can download.

Both Fusion and Parallels have an integrated mode, where Windows apps appear on your OS X desktop and can be run directly, without switching to the Windows view.

If your MacBook Air has only 4GB of memory, then running any virtual machine will be performance limiting, because OS X needs more than 2GB, the virtual machine needs its requirement in RAM, and then your Windows guest must be configured with sufficient RAM to run faster than a snail. With 4GB, your better performance option would be Boot Camp.

With 8GB of memory, then you have a realistic choice between Boot Camp (if you want exclusive dual-boot for performance and compatibility), or one of several virtual machine products. All of these have support forums.

• VirtualBox, 4.3.28, (Version 5 soon) free, Oracle Corporation. (what I use with Windows 7 and Ubuntu guests)
• Parallels Desktop 10.2.1, $79 (Sale now: $59), Parallels, Inc.
• VMWare Fusion 7, $69.99, (Sale now: $52.49), VMWare, Inc.