When you open Flash CS3, you will have several options for Actionscript Language versions. For reasons of simplification we will be using Actionscript 2.0 instead of the new 3.0. 3.0 seems to be more for professional developers than novice and beginning users. It requires a fair amount of programming knowledge.
Once the new 2.0 file is open, setting the stage size is first order of business. The stage is the area that will be published in the end, its important that your stage is the size you would like have your flash site appear on your html page. Frame Rate will also determine how much data your flash animations will require to play back. The higher the rate the smoother the animation but larger amount of data to load. Its a balance. Starting high and then lowering is a good way to go.