Okay.
As is:
stereo outs from TV into sub, sub connects to satellite speakers (sub also acts as amplifier for satellites) via whatever provided cables. You don't get 5.1 with this option.
If you get the Logitech Z-5500 Digital or equivalent:
digital from TV into sub (they need either to both be coax, or both be optical), sub connects to satellites via whatever provided cables.
If you get a prepro:
digital out from TV into prepro (again, needs to be the same), pre outs via RCA into 5.1 analog inputs on sub, sub to sats via whatever.
If you get a receiver:
same as prepro, or, just use the receiver traditionally -- speaker outs to speakers, sub out to sub -- I wouldn't recommend this if the documentation with the 5.1 system says not to, as that means that there's some special processing that the sub does, which your receiver won't.
I tried to make this post as short as possible so that I didn't muddy up the works, so feel free to ask questions.