I Wanted to start doing balloon decorations and other things, but I never did a design in my life and people have already booked me for events.
Are there any classes I can take, or seminars?
You seem to be a good sales person, if you've already got yourself your first decorating job. Well done. :-)
A good first start for you would be to look through the step-by-step instructions here on Balloon Decoration Guide. This should help you to create balloon bouquets as centerpieces, as well as simple columns and arches.