I would say it's easy enough to build your own PC that you could watch a few demonstrations on youtube and be very likely to succeed on a pile of selected parts put in front of you.
The main thing to keep in mind is what type of connection each component makes to the motherboard. If you choose a certain CPU (processor, instruction core, whatever you call it) then you want to look at the specifications for that CPU and see what kind of Slot or Socket it's designed for. You then must look for a motherboard that supports that type of interface connection.
When you choose a hard drive, you must check the motherboard specifications to see if the drive has the same interface type (ATA or UltraDMA or EIDE are all different terms to describe the same interface, whereas SATA has a different connector and different cables, and SCSI is yet another common interface). You must make sure the hard drive or the motherboard comes with the correct cable for that connector type, or otherwise order the cable.
When you choose a video card, you must ensure the motherboard has a connector of the correct type (PCI, AGP, PCI-Express) and take note of what kind of video output interfaces it offers (DVI, HDMI, VGA, S-Video). When you choose a monitor, you must go back to that note and make sure the monitor supports that video type. Once again, check the monitor and video card descriptions to be sure they come with the correct type of cable.
For choosing RAM, I would use
www.crucial.com as a resource. They should allow you to enter in the make and model of the motherboard you chose, and generate a list of RAM products which are known to be compatible. Most online computer parts stores carry some Crucial RAM, but you can also choose another brand with the same interface (SDRAM, DDR, DDR2, etc.), same number of pins, clock speed, PCxxxx rating, and CAS latency.
And definitely check out the three websites that TheLoveBandit referred you to.