pull the seat belt across your lap so you can just get it buckled (but don't buckle it yet), twist the belt 3-4 times and then buckle in. This will keep the lap belt from untensioning and hold you in somewhat better. You can also try to engage the shoulder belt by jerking it sharply, but I could never get the trick to work very well.
Baring that if you are not using your left foot to brake plant in on the dead pedal and push yourself back into the seat.
For your first event, don't worry about doing too much, just try to get your brain up to speed. It will feel VERY fast at first. Concentrate on looking ahead and don't stare at any cones or you will hit them. Try to pick one or two things to work on per run (braking, looking ahead, etc.), if you have a region with a novice program take full advantage. Ask all the nice people at the event for advice and rides, they will generally be happy to help (the words "I'm new at this" or "This is my first Autocross" or "I'm a novice" will get you more advice than you will know what to do with).