JS To Select An Option In A Combo Box

How do i select an option in a combo box? i have tried looking on the internet but dont really know what i should be searching for. what i want to happen is that when a function is called
a line of code will select a specific option in a combo box. something like:

//excuse the dodgy syntax

function select_combo() {
cb_1.select[3] //where option 3 will be selected (or 4 if first index
is 0)
}


the combo box would be something like:

<select>
<option>First</option>
<option>Second</option>
<option>Third</option>
</select>


The function would change the selection in the combo box.