I really don't know much about the business itself. I do know alot about business in general, tho. A few of the girls in my office like to plan and coordinate events, and have done a wonderful job on the few jobs that they have done for family and friends. My best guess and advice would be to do some homework, shop around and find the places in your area that have the best bargain on decorations and such. You want to be able to provide high quality at low cost and that takes time and effort sometimes to locate.
I would start by helping out family and friends. Maybe run a small ad in your local newspaper and go from there. Figure out what you would want to charge based on the type and size of event, whether you want to calculate your earnings by an hourly wage or a set fee. Again, I think if you have to find the best deals with decorations, renting tables/chairs and such, start there. Put extensive time into seeing what other companies like your idea are already available in your area. Call them, find out their fees, their requirements and what they provide. They don't have to know you have your own motives, you can ask as a possible customer.
This is what I would do if I wished to pursue something like this. I am not creative, don't have the time, but like I said I have seen a few girls in my office do gorgeous work thru their joint efforts.