how to create such a function??

Hi there,
I want to create a function and a trigger, but I don't know how... :(
I need something that would work when I update the table 'stock' , by insert some integer value into 'sold'. Then the integer value in field 'stock' has to be decreased by that inserted value.

example of my problem:

-table 'stock' before updating 'sold' field

ticket_id | stock | sold
1 | 30 | 0

-how table 'stock' should look like after: update stock set sold=2 where bilet_id=1;

ticket_id | stock | sold
1 | 28 | 2

I have to create something that would work like that, after each update of ticket_id defined rows.

I know that you guys can help me with that... apreciate any help which will be priceless ;-)

iron