Gauging Wire for 12V Parallel Circuit
You'll need to take a look at an ampacity chart.  They show the maximum current a conductor can carry based on how hot you want to allow it to get (i.e. before it starts melting insulation).  There are several caveats:
1: The temperatures in the chart are all based on a certain ambient temperature, usually around 30C.  If you plan to operate above this temperature - and you should, 30C is only 86F - you'll have to correct down.  Some charts will have a correction factor on them so you can find the new allowable current.
2: Since no single chart could account for all the possible physical configurations of bundling, sheathing, or otherwise covering the wire, many of them only give values for a single wire in free air, which is the best case.  As soon as you start zip-tying the wire in with other conductors or putting it in conduit, it will be harder for heat to escape from your wire, which reduces the amount of allowable current.  You'll see that the linked chart also gives values for a maximum of three wires bundled together.
3: While the values you will see on an ampacity chart are based on relatively conservative assumptions, remember that they are conservative assumptions for the best case scenario.  Give yourself reasonable buffer.
4: Since I'm a connector and cable guy, I'll put in a note here that you should not neglect your connections - to your battery, your switch, or your loads.  A bad connection can cause hard to diagnose problems or create a dangerous situation, especially once you get out of sub-Amp territory.  Good solder joints and good crimps are the keys to success and it only takes a little practice to get good at them.
Edit: You'll notice I didn't say what size wire to use.  That will depend on your specific wire and what it's insulated with, etc.  But you can see from the chart that 18 AWG is too small for at least the main lines going to and from the battery.

Ampacity chart from Blue Sea Systems