If you want to see what an init.sqs looks like download the Tutorial Mission in the Ed Depot - Tutorials - Getting started. Use the one in there as a template.
snYpir wrote an excellent tute on how to use objectives and init.sqs, which is also in getting started.
One of the secrets with all script files is to make sure the name is init.sqs and not init.sqs.txt, which can happen if you are not careful.
The underscore was meant to be an underscore, but it's just a variable - you can call it pretty much anything you like, it doesn't really matter. You could write
endvariable=true
exit
and use endvariable in your trigger. Btw it's good practice always to end a script with exit.
Have a look at the tutes. Come back and ask if you get stuck.