There is no fun in DIY if you don't assemble the PC yourself. If
you gonna get some guys to assemble it for you might as well get an
OEM like Dell or Acer. Don't be surprised you will go into some
trouble if someone else install everything for you with all the
separate hardware you chose. Get an OEM you are still backed by
warranty and aftersale service.
For complete noob you have to get to know all the processors and
the mainboards with the various chipsets. Learn the form factor and
how each processor type fits into the board. Getting the incorrect
pin count means a misfit. For graphics card its the same but you
should not go wrong, after all there's only 2~3 sizes to choose
from - AGP, PCI-e or PCI2.3. Hard disk SATAII and RAM DDR(2) should
be the trend now while most card should support larger screens and
wide resolution too. PC case sizes are still generic at ATX while
the ROM and Floppy still stays. You do have to take note of the
output power pin count for the PSU to choose from as the correct
type with enough power will then be able to flare up your system
and run it steady. As for sound you can add a card but for the
average user, the onboard sound chip at the mainboard should be all
that is needed.
OS is the other important thing and for a first build it usually
takes up a portion of the budget.