Welcome to the forums. The first thing to do is relax and smile.
As long as you know which end of a medium phillips screwdriver is the business end, what you want to do is easy, as long as you take the time to figure out what you want to do with it learn a few things about which parts that will get you there. If you want to do some serious gaming, you'll spend more for a video card, and you'll want more RAM and a power supply hefty enough to support all that and a faster, more expensive CPU with better cooling.
If you're not into gaming or any other immediate apps that demand power hungry components, just get a decent case and a good, reliable power supply, and build a comfortable machine for less than you're willing to spend. If you have a good base setup, you can always expand its capabilities as you learn what they are and what kind of extra power you need for them. Then, you can either upgrade what you have or sell it and get the new rig you need... then.
One more thing -- Check out Hot Deals and other deal sites for specials on parts. Recently, I've seen good DVD burners for $20, 2 GB of DDR2 RAM for $60, large hard drives for under $25 - $35 and more.
Take the time to feel comfortable with what you buy, and you'll win the game. You've come to the right place for the info.
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