Default User ID

Hi, is there a way for me to get a field from a table and set that as a default value for another field? What I want to do is get the ID of the user that's currently logged in and use that as the default value for another field. I plan to do this in the delegate class for the table, like this,
function User_ID__default(){
return '0001';
but I don't understand what I'm supposed to return. Thanks for your help.
function User_ID__default(){
return '0001';
but I don't understand what I'm supposed to return. Thanks for your help.