First, welcome to the forum---nice to have you join us. Now, about that wiring. A common practice is to run a heavy wire called a bus to carry the electrical load with minimal power loss from resistance, and then take smaller feeder lines off it. Think of it in terms of plumbing: one large water main runs down the middle of the street to supply everyone, and they get their water by tapping into it with smaller pipes for each house. So, for the bus, you might decide to run a heavy wire to an attachment block under the center of your layout, and then attach feeder lines from there. I'm going to give you the address of a thread we recently had discussing this: http://www.modeltrainforum.com/showthread.php?t=3759
You'll see two things, reading it. First, this site has a lot of confusing people who disagree over things as simple as wiring. Second, there are a lot of "ifs" involved, when you start talking DCC and other specifics.
Let's start simple and say you have nothing like DCC, just a simple layout with a lot of turnouts. My suggestion is that you wire your track on it's own bus, and then take some feeder lines out to the most distant parts of your track from it. If you find your train slowing in some areas, add feeders to those areas. Then run a second buss for your accessories---all those turnouts. Feed them from your second buss. Others will probably give you better advice, but I'd say 16 to 18 gauge wire for the busses and 20 gauge for feeders to each of your turnouts. You don't want the turnouts competing directly with your train for the power: you end up with turnouts that don't have enough power to complete their movement or a train that staggers when you throw a switch.
Again, I'd suggest waiting till you get two or three more replies---people here are pretty knowledgeable, and most know more than I do. Best wishes on your layout, and show us some pics as you're working on it!