Putting together a PC is no problem. It's kinda like LEGO, and the parts will only fit in one slot and only in one direction. I've been building my own PCs since 10 years ago. Only build GOOD PCs, because for a cheap PC, i can't do it cheaper than Dell, and neither will you be able to.
I'll leave a warning here though. The parts are quite sensitive, so discharge your static electricity before working on the computer. Trouble-shooting will be a big part of building your own PC. Lastly, turning on the computer without RAM chips installed will burn your graphics card.
Generally, anything you burn/damage/destroy can be replaced on warranty. At least that's how it works here. I've destroyed hard drives, mobos, GFX cards and more doing this, so if you are not sure if you can replace it, don't go experimenting.
Don't scrimp on the power supply. The cheap ones can be really useless. Spending more on the motherboard doesn't make sense to me, the expensive ones and the cheap ones seem to both last about as long - only buy the expensive ones if you need the extra doodads like RAID or SLi. I've gotten quite used to swapping my motherboard now and then when it burns out.