First off select a menu that you are familiar with,
Look at th kitchen you are going to have to cook in and for this you will have to determine which foods can be prepped the furthest out and make a chart, figure on a sit down about 5 oz of protein per guest, up to oz for a buffet since they will be allowed to take all they want, 4 oz for each veg, for your own knowledge a can size known as a Number #10 can holds about 25 portions of vegetables (its is the large can used in commercial kitchens/restaurants. Take each step individually and once you seem to have each step planned out combine them. You should be able to figure out cost, your shopping list, schedule for prepping your food etc. If you have any further questions please contact me through my web site. I have prepared meals from 1 guest to 4,000. Rest easy it will be ok.