You're not the first: one of the tricky things about getting started in mission editing is the question of Objectives.
Conceptually, Objectives are associated with triggers. Triggers detect when something happens and then "trigger" the consequences. With an Objective, the consequence is usually a green tick on the Plan page of the Briefing and possibly the end of the mission.
But what is the trigger detecting? Well whatever you like. A particular unit(s) being not alive; the player being in a particular place; whatever you like really.
For the technical aspects read snYpir's "How to use Objectives and init.sqs".
I wrote something about this the other day ... wait ... read
this.