Populate a value using a trigger

Hi,

     I am trying to write a trigger which would populate a column for the table where I'm inserting.

Example :- table A(x varchar,y num,z num) is there.

I insert x and y through an insert script but z should be populated by a trigger. Where I can assign a value to  :new.z with the help of other tables.

When i tried the same it says - incorrect syntax near ":new"

Please help.

Thanks in advance.

Athrey