You can start getting Linux distributions (there are multiple of those) from here (at LQ) for example. On the right is "main menu", there is a link "download linux", see that. You'll need an empty cd or an empty dvd to burn the iso image (as image, not as regular data) onto, and before burning an installation disc I suggest you grab one of the Live distributions/discs that you can boot and try Linux with before actually installing the operating system.
- there are multiple different kinds of Linux' (called 'distributions'). Basically they have a Linux kernel and some software (mostly GNU software), but the kernel and program versions may differ, they may have different sets of software installed out-of-the-box, they may be tuned in different ways etc. Basically "Linux" means the kernel, which is quite the same in all of them (but patched, for example, so not exactly the same), but generally if you learn to use one Linux distribution you can quite easily learn to use the rest (and Unix systems as well) because they are so similar.
- usually you get more programs using a package manager
program of the Linux distribution you install. It's a program that has a list of available programs that you can simply point&click-install (or something as easy as that). Or you can download a suitable format binary package for that distribution from the web and install that, often by double-clicking on it or running a command to install it (documentation is found for each distribution on how this is done), or if you can't find anything else, you can always compile programs from source code if you have compilers etc. installed.
- updating programs is usually also done by the package manager program; on some systems (like Fedora or Ubuntu for example) you have a nice little icon on the menu bar that blinks when you have updates available and asks you if you would like to install them. A little like in Windows.
- device drivers, or many of them, come with the distribution (usually compiled into the kernel or as modules that are loaded when needed). In some cases you don't have them so you might need to manually install them (or compile into the kernel), and in some cases if there exists no driver (for Linux) for your device, you may be able to use the Windows driver (for example with wireless ethernet cards you can use ndiswrapper
program to use Windows .inf driver files, if native drivers don't exist)
- system requirements depend a lot
on what you want and choose. A Linux system, if you want to make it that way, can mean you get a web server installed in less than 8MB. On the other hand if you don't like to do a lot of work but just grab one of the most 'famous' distributions you'll probably end up using 2-5GB of disk space for a whole range of programs and stuff. Linux can run on very low memory and with slow processors, but if you want to have a "modern desktop", I suggest you have 256MB of RAM or more and a decent cpu. If your Windows runs on some hardware, then Linux runs on that too -- but not necessarily vice versa
Take a look at distrowatch website also.
EDIT: nowadays if you choose to use a "desktop Linux distribution" you can live with graphical desktop and often don't need to take a look at the console if you don't want to. Nearly every task can be done using graphical user interfaces (like you do in Windows), but if you want to, you can take advantage of the console; after all, X (The graphical server) is just a program of one kind, and not the only way of using Linux. Look at the linuxcommand.org
website for a start-off with console commands; sooner or later you probably will want to know how it works, so it doesn't harm you to know the basics -- removing and moving files, editing files and so on. If you get into it, you can do more with your console than with your X programs.