Buying (or possibly building) a new PC

[Okkusenman_110Million]

So I'm fed up with my current PC. Takes forever to start up, lags randomly at times, and the graphics card is just awful. So I've decided to buy (or build) a new PC. I have absolutely 0 knowledge of hardware, so I was wondering if anyone here could enlighten me? What do I need to look for to get a good gaming PC? Buying an Alienware is the last thing I wanna do, since I'll probably be paying more for the brand name rather than an actual good PC (compared to one that maybe someone could build for the same amount of money) but if I have to, I will. Thanks in advance!