set field value with workflow action script after 'go to record' action

Hi,
I have no scripting experience but I was thinking what I like to achieve shouldn't be too complicated.
all I like to do to create a workflow action script and after I used the 'go to record' in my workflow able to set a few field on that record.
(my workflow running on a custom record (entity bank details) and moving to vendor record but I need to set a field value on the vendor record)
I would appreciate any help.
I tried this (using the comment field as test field)
script:
function setfield()
{
var rec=nlapiGetNewRecord();
rec.setFieldValue('comments', 'test_comment');