The main thing is the orders. They usually will state things OK, though in Stock, they can be ambiguous at best sometime. If they tell you to proceed to an area and receive further orders there, that will be a "Sink" mission, and it will sometimes have a tonnage listed and what type of ship to sink, such as Warship or Merchant. If they say to Patrol an area, see if they don't tell you how big of an area 100 nm is not unusual. If such is the case, they will usually tell you how many days or hours to do such. You should use the compass tool on the NavMap and draw yourself a circle no more than 50 nm in radius (100 nm diameter). Once you draw that, start zooming in on the center of the star and your circle, and again use that drawing compass tool to move your circle's center point to the center of the star. Once you travel to that area and enter your circle, don't leave it until the mission completes. Somewhere on the site, someone has a rather comprehensive post about completing all of the different Objectives - found it!
Beery - Guide: How to Complete a Mission
You could also ignore the 'ordered' Objective, so long as you come home with some prizes, which you have. Now, the game, for whatever reason, cannot tell when you've disobeyed a direct order. All it can see is if your renown points go up, or did they go down... I always play it like an order is an order, and that the Navy Department would not have given me said order, potentially putting me and my crew at risk of death, or worse - not forgetting the taxpayer's good money spent on the equipment entrusted to our care - just so that we could whistle Yankee Doodle Dandy in Empire Waters... We have a job to do!
Dontcha just love it?