Becoming a Gamemaster for the first time can be quite a daunting process, the 3den editor is very complex and there's a lot of options on there that won't make a lot of sense to you at first. The tutorial below runs through the basics of making a simple mission with some simple objective types. It should give you everything you need to create a starter mission and then go on to use your own ideas!
The YouTube videos above give you some additional tips with the 3den editor so that you can bring your missions to the next level, while entirely optional, the the information in these videos will be a great asset to any Gamemaster.