[Desire HD General] bananagranola:
My spiel usually goes like this (sorry for the massive wall of text):

Start with a "user-friendly" distro. My favorite is Linux Mint (better desktop environment in Cinnamon, with the option for the less-beloved Gnome 3), but many people use Ubuntu (with the also less-loved Unity) and Fedora (with only Gnome 3). Or there's OpenSUSE with KDE.

I'm mentioning DEs because DEs and starter software are what everyone sees in a distro, and any starter software can be easily downloaded and installed with the distro's respective package manager. But most come with, say, LibreOffice (which can read and write .doc and .docx, along with other Microsoft Office formats), a PDF reader, a music player, a file manager, and all those other basic things. Which, of course, are all replaceable with something you like more.

Now. Use the terminal. Use it lots. Use it for everything possible: configuration, settings, installation of packages, file management, text editing (I use vim), etc. Google for commands you don't know. But the advantage to using a "user-friendly" distro is that, when you can't figure it out in the terminal, you can do it easily in the GUI. And then, feel free to stop here if you like your computer the way it is.

But. When/if you're comfortable in the terminal, and when/if you still like Linux and the terminal environment, do some "hardcore" distro research. I want to make clear here that I'm not saying any "type" of distro, like "user-friendly" v. "hardcore," is better than another. But it is easier to force yourself to do everything in the terminal when, well, there's no GUI available (Arch Linux, my current distro). Or when you need to compile the thing from scratch (Gentoo, Linux from Scratch). In a shameless plug, I should mention that the Arch Wiki is an absolutely essential resource and one of the best (if not the best) Linux resources available. Choose your own distro. You may try several before settling on one. (Note: do not post on the Arch Forums until you figure out their rules. They are not friendly to questions that are asked and answered, open-ended, or several other classes of things, but are incredibly helpful within their bounds. I had an odd transition period from there to here.)

I keep emphasizing the terminal because it's where the power of Linux resides. I say this because just about everything in Linux is configurable, but not everything is configurable from the GUI; many things can only be done in terminal. And commands like grep and sed, and shell things like | (pipes), are incredibly powerful - but you can only harness their full potential in the terminal. And eventually you realize that, for you, it's become faster to do things in terminal than in GUI. Even things like basic file moving and copying.

Look into Linux-specific applications that do things Windows doesn't or can't offer. My laptop is small and not very powerful. So I use a lightweight tiling window manager instead of a full DE. Most functions have applications available that use lightweight terminal frontends. Now that I'm used to my terminal apps, I use them even on my higher-powered desktop, even though it can handle Gnome/KDE and more heavyweight apps.

When installing apps, be aware that Linux distros come with something called a package manager. Like how Android uses .apks, distros package their software in their respective packaging formats and install it with their package management system. This tracks the files of the software; most package managers handle dependencies without your intervention, and they make uninstallation easy. All software should be installed through the package manager; if software doesn't come in the package manager, you can usually learn to make your own fairly easily with the aid of Google (and this is actually a good starter thing to learn). Some distros come with GUI frontends for this. I would advise learning to use the CLI version instead.

Also, I should note that since you're building a computer from scratch, make sure to consider the compatibility of your hardware parts with Linux. Some are significantly better than others; for example, ATI graphics cards are notoriously poorly supported, but Nvidia has a pretty good Linux reputation. Bluetooth adapters can also have issues, either dongles or on the motherboard. I actually bought a Bluetooth dongle separately because I wanted a motherboard without having to factor in consideration of bluetooth Linux compatibility.