Make form field active / inactive dependent on previous choice

[eluser]DannGrant[/eluser]
Hi guys,

I'm creating a form where the user has to choose from a list of items in a dropdown box.

If they choose 'other' I then want the field underneath to become active and allow them to type in their 'other' field.

Honest answer is, i don't even know where to start.

Any pointers?