Username or EmailPassword
I posted a flip response earlier, but, to be serious, think about going the barebones route. A number of online vendors enable you to select your preferred components, and will then assemble, at nominal cost, what's called a barebones box for you. Typically, they install the power supply, fans, and motherboard, as well as connecting the power leads to the on-off switch and the like. (If you're going to make a mistake that will fry your system, it's the wiring that'll likely get you.)
That leaves you to add your drives, your sound and video cards, and any other little goodies you want.
This is a lower-risk way to go that will tech you just as much as doing it all. Unless you really think screwing fans into the inside of a case is a thrill, give it some consideration.
As for the rest, buy the best you can afford so you can do what you want to do, but don't buy what you won't use. If you need cutting edge hardware, buy it, but realize that the price you pay today will probably drop precipitously in the following weeks. If you're a gamer, pay attention to sound, video and monitor. If you're a coder or a web person, understand that you spend all day looking at 2-D displays and go for a good 2-D video card. I'm in the 2-D category and recommend Matrox cards. I've used them for several years. Take a look at the 450 or the 550.
Remember, OEM and white-box parts are cheaper, but you won't get any instructions or manuals.Some times current manuals are available on the web, sometimes they aren't.
Don't buy the cheapest case. Consider an aluminum case -- they're supposed to keep the internal parts cooler. (Who knows if they really do. My aluminum Lian Li case does feel cool to the touch.) Buy and install enough fans. Better to spend an extra $30 on a fan than to bake the system. Don't buy a no-name marginal power supply. Get a brand name supply that can produce more power than you need. You never know what you'll add on later.
Assemble it in a place with lots of light. Little screws and little jumpers are easy to lose. In addition to the Phillips screwdriver and a pair of needle-nose pliers, grab a pair of tweezers to hold those little screws steady and in place for your screwdriver.
When it's built, put your new toy someplace well-ventilated. If you can avoid it, don't put it on the floor wedge between the side of a desk and the wall.