wirng sub and amp

First...  What amp are you using?  That will tell you how big your power and ground need to be.  If it requires more power, then you'll need a bigger gauge.  Until you tell us that (along with the amps fuse rating), we can't tell you how big your wire needs to be.

Second...  The speaker wire running from your amp to the sub doesn't need to be 8 gauge.  I've used 12 gauge before, but it doesn't even need to be that big.  You'll be fine with something like 14 gauge or 16 gauge.  You won't be able to hear the difference.

Third...  Wiring from your amp to your deck can be done with something as small as 18 gauge.  Your using this for your remote turn on, right?  If I'm correct, then use something like 18 gauge or 16 gauge (if you want).  Just make sure if you use the !8 gauge that you make sure it doesn't get cut or anything.

Fourth...  Make sure you have RCA cables.  I don't know if you have them already, but i decided that I should check.  They don't need to be expensive, but don't buy crap ones.

Now I don't want you to take this the wrong way, but I am getting the feeling that you don't really know what is going on with the whole installation thing.  You might want to invest in some pro installation or work with a friend who knows what he or she is doing.  I'd also say that you should buy a pre-packaged wiring kit.  Make sure that the power and ground cables are right for your amp.  If you do this, then you won't have to worry about all the little pieces you'll need.

If you have anything else, then feel free to ask.