They are relatively easy to put together. Just follow this simple plan, and read any instructions you get with your parts carefully:
Case: get it ready, take off both side panels, sort out power cables, clip in the back plate for your ports, attach any extra fans etc..
Motherboard: rest it on something soft, like a piece of foam. Attach the CPU/fan, Ram then fit the MB into the case (Taking note of any extra posts that are needed, some cases differ)
Knock out front plate where your DVD drive is going and put that in.
Then put in your HDD/s.
Knock out back plates where your Graphics card, PCI express card(if you are going to go wireless for internet) are going and attach them to.
NOW plug in all your cords and the SATA cables that go into the DVD Drive and Hard Drives/s. Make sure none will get caught up in any fans. This can be a little tricky for a beginner. Follow your Motherboard's user manual. It will show you where each cord needs to be plugged into.
I would then recommend you fire that beast up before you cable manage, just to check you put everything where it should be. Then do the last step.
CABLE MANAGE..this is tidying up all the loose cables and tying them all together (Plastic cable ties are the best, NEVER use metal) DON'T tie the SATA cables in with the rest. Leave them free.
Put your side panels back on and now you should be ready to install software and drivers (for the hardware).
Of course there are many more tips I could write here but I don't want to scare you off building your own rig. It is quite fun and very satisfying when it comes together. Plus you would save a lot of money putting it together yourself.