Well if you plan to piece it together over time, there is no problem with picking out a few things now that won't really change no matter how much you learn later. Like, you're 99% going to want an ATX case, whether it be a micro ATX, mini, mid, or full-tower ATX. ATX is the current standard for motherboards so the only real mistake you can make is to buy something like a BTX case. Most cases will be mid-tower ATX because they hold plenty and still fit under a desk. Newegg has a huge selection and a ton of manufacturer rebates. I picked up my Cooler Master Storm Scout for like $60 there.
Things to consider will be aesthetics and price (obviously), number of and location of cooling fans, wire management capability, is the interior finished/painted or bare steel, number of 5.25" and 3.5" drive bays and how they operate (screws, clips, rails, etc), and then extra features like LED light controller, front USB 2.0 or 3.0 inputs, digital display, does it have a carry handle, does it have a side window, and other stuff like that.
And this is another site I like, which is good for comparing graphics cards:
http://www.hwcompare.com/
