It's really easy.
First, choose a distribution (known as a distro) that suits you. That's by far, the hardest part, as there are hundreds to choose from. However, in order to get the list to a more presentable number, here's what I suggest you look at:
Linux Mint - an excellent Windows like operating system, that will be fairly familiar in use to any Windows user.
Ubuntu 10.10 - Note: not Ubuntu 11.04, which sadly is buggy. Well supported and possibly the largest user base for a single system.
OpenSuSE- an enterprise strength distro, with some very solid background code, in an easy to use interface.
Accept, Linux is not Windows. That may seem a very obvious statement, but time and time again, I see Windows users expecting to maintain the system exactly the same as they do in Windows. Don't even think about it! Linux is Linux and you have to accept you are going to do things differently. Once you're used to it, in many ways it's a lot easier.
You can find here, an article I wrote for my local Linux User Group about four years ago, but is (mostly) just as relevant today. It gives an overview of what to expect and also provides some links to other relevant articles. Note particularly, "How to burn an iso file" if you've never done it before.
You do need to consider what hardware you are going to run this system on. The latest distros, (while not requiring Windows bloated specifications), do need reasonable specifications to work properly. If you're computer has a low specification (particularly RAM), then Puppy Linux may be a good choice.
Please come back here if you need further help.