Check out snYpir's excellent "How to use objectives and init.sqs" in the Editors' Depot ... tell you all you need to know.
If I remember rightly the key points are
"1" Objstatus "done"
"1" Objstatus "failed"
but don't quote me. These lines would go in the activation field of a trigger, the condition field of which would relate to the objective. So if the objective was to destroy a tank called goodnightVienna then in the condition field you would put
not (alive goodnightVienna)