if you take a look at your trigger types, there are some called 'end'.
create a trigger, 0 diameter, type 'end'. in the 'conditional' field, put "end_mission" or something similar.
that
end_mission is the name of a variable. when it's false, the mission continues, and as soon as it's set to true, the mission ends. if your next question is 'how do i set it to true?' then you really need to get yourself over to the editor's depot and take a look at the beginner's tutorials. it may seem the long way around things, but you'll appreciate it afterwards. i certainly did, and i almost never read the instructions...