As far as what to know about electrical:
You have voltage, current, power, and resistance. And they are all related to water, Voltage (measured in Volts DC) is like the speed of flowing water mph, current (measured in Amps) is the amount of water flowing like gallons per minute, and resistance (measured in ohms) is just like it says resistance to the flow of water. And power (measured in watts) is the Voltage * the power, or it is just like the power of the water.
And the math there are a few equations you need to know
V=IR or voltage = current * resistance
And Power = Voltage * Current
From those 2 equations you will be able to figure out almost any of your problems.
So letís say you have a light bulb (or headlamp) that is rated at 50 watts, you want to figure out what size wire to use, so you put what you know into your equation,
50 watts = 12 volts * Current, so you solve it and find the bulb has a resistance of 4.16 ohms
You apply what you now know into the other equation
12volts= I * 4.16ohms, you solve this and find that you will have a current of about 2.88 amps. And by looking at the table below you will see that you would only need an 18 awg wire.
Things to remember, when hooking lights you have to take into account each light, so if you have 2 50 watt lights then you will be drawing about 6 amps not 3 and will have to use at least 18 awg wire. For safety I try and use a larger size wire then needed to be safe.
For a table of wire size vs current or power go to http://www.rbeelectronics.com/wtable.htm
A good source of info is http://www.the12volt.com/
With thanks from replies to my posts from Mike Hippert (101) above and DJ Menasco (201) the attached