Once you've read snYpir's tute, read messiah's tute on camera.sqs, it will make your life much easier.
Cutscenes before the mission come in two flavours. The first is the Intro as described by Ace. (The drop down box he mentions is about halfway down the right had side of the mission editor map screen.) The Intro plays before you see the Briefing. Alternatively/additionally, you can have an ordinary cutscene at the start of the mission.
Similarly, the Outro plays after the debriefing screen.
Whether you are making an Intro, Outro, or cutscene the process is essentially the same - you set up a scene with units, vehicles and objects, then you set up the camera view either in a trigger (not very good) or a little script (good). The tutorials (there's a whole sub-category on this) will help you to create your first cutscene, if you get stuck search the FAQ and the forum. If that doens't work post a question on the Cameras and Cutscenes board.