You'll want to use stranded wire (as opposed to a single solid conductor) for auto/motorcycle uses. In America, we use "AWG" (American Wire Gauge) as a measuring standard - the higher the number, the thinner the wire. 18 AWG wire is thicker than 24 AWG wire. I'm sure there's a metric standard as well, but I'm afraid I don't know what it is.
Here are a couple of specs I borrowed from a handy online chart:
10 AWG wire – max 55 amps
12 AWG wire – max 41 amps
18 AWG wire – max 16 amps
24 AWG wire – max 3.5 amps
The two-conductor wire you can pick up in a hardware store is probably thicker than you'll need for most motorcycle applications. Plus it can be hard to differentiate the two conductors if they're molded together (like an extension cord). For AC household work this generally isn't a problem, but motorcycles are DC, and bad things can happen if you mix up positive and negative.
If it was me, I'd go to an auto parts store and get a couple of spools of 18 or 20 gauge stranded wire – one red, one black. That should handle most accessories up to about 200 watts. You could go smaller if you're not hooking up anything that big.
Conduits are nice to keep things organized and tucked away, but I've usually just used tie-wraps and followed the routing of the other cables. Happy wiring!