Disabling And Enabling A Textbox?

I have not used JavaScript in a good long while (i'm a SQL guy by day) and am running into what I consider to be a simple problem. However, i've done LOTS of research on my problem and not able to find resolution on my own.

I've got radio buttons - one for Male and one for Female. I also have a textbox which is disabled by default when the HTML page loads. When the FEMALE radio button is checked, it should enable the textbox so that the end user can enter text into it. When the MALE radio button is checked, the textbox should remain disabled. The HTML page is calling a JS function which is inside a separate script, see code below.

Code:

function enableTextbox()
{
if (getElementByID("F").checked == true)

[code]...