Hey,
I will try and give you a hand.
First off the Processor, nowa days almost anything Intel is better than anything AMD so if you see the letters AMD i would look for an alternative.
Looking at Intel processors a little further, you have 3 ranges worth considering i3, i5 and i7. The higher the number the better the processor.
Next to the processor you may also have whats called a clock speed (should be measured in Ghz). A full example would be "Intel i5 3.2Ghz". First look for Intel then look for the highest "i" range in your price bracket then look for the highest clock speed.
Now the memory.
More is better, you don't really wan't to be buying a machine with less than 4Gb RAM nowa days. If it says DDR2 walk away, it needs to be DDR3. In further detail you may be given the clock speed 1600Mhz is an average, the higher the number the better. . . .
Someone else on here can advise you about graphics cards as that is NOT my strong point.
Other than this you want to look out for some key technologies, the words USB 3.0 will be a plus, so will an SSD hard drive, Blu Ray drive would be nice.
But essentially what makes your computer tick is the Motherboard, CPU and RAM (memory), (and graphics card for gaming). The motherboard is a complex one and essentially if you have a good CPU and good RAM we have to trust that the manafacturers would have put it on a half decent board
? wouldnt they
)
Hard Drive size is only for storing your photos / videos / documents, if you are an average user 500Gb is fine (don't be sold on a computer JUST because it has a 2,000Gb hard drive, sounds impressive but honestly an average user probably doesnt need more than 500Gb).
That's pretty much all i can think of right now. . .
**note that the above are only guidlines there are cases and situations where these guidlines don't make sense but you should be safe in general.