If you want to use relays to control the high/low beams, as Danny suggests, you'll need only one relay - a single-pole, double-throw (SPDT). Double-throw relays have five contacts. The ones with only four contacts are 'single-throw'. Just make sure you use a 40-amp relay so it's better able to handle the power for both headlights.
The little switch that you mount in/under the dash to control the relay need only be single pole, single throw (SPST). And since it controls only the relay, it doesn't need to be rated for heavy current.
When the dash switch is 'off', the relay is 'off', too, and the relay routes the power to the low beams. When the dash switch is 'on', so is the relay, and it routes power to the high beams.
You turn the lights completely off with whatever switch you're using for that now.