You don't have to buy a PC with Linux already installed per se.....
You can go to
Distro Watch for info, where 100 of the most popular Linux flavors are rated. There are more than 100 versions of Linux and most are free.
Ubuntu is the flavor I chose because they have massive
Support Forums that are very helpful answering any newbie and highly technical advanced questions you may have. Mint is very popular now and is similar to Ubuntu. The Ubuntu site will give you all the info needed to download Ubuntu and create a 'Live CD' that will enable you to run Ubuntu off that CD to give it a 'test spin' on your PC without changing a thing on your PC. Then if you like what you see you can install it and it will run even better once installed on your HDD.
I went with a dual boot setup at first so I could learn Linux at my own pace and still have Windows to fall back on when needed. With a dual boot you have 2 completely independent OSs on your PC. Ubuntu site gives you all instructions needed to setup a dual boot which I prefer. Or you can just install Linux on an old PC you may have. Linux runs equally well on any PC old or new. Now after using Linux ~6 years, all self taught, I don't need M$ anymore for anything!
Ubuntu comes by default with many programs, all you need really, including a complete Office Suite of Libre Office which replaced Open Office a couple years ago. Plus there are several thousands of other software programs/apps available in several Linux repositories all FREE.
Linux for the most part is a FREE non-profit organization made up of altruistic folks who believe the Internets should be FREE to all!...:thumb:
However there are a few commercial Linux distros that are trying to make a buck and will charge you to use them. I just pass on these pay versions.