Deciding on a theme is a good starting point. Determine how many guests you plan to have and how, or if, they will be seated. Look at your space and decide where you think flowers could be displayed for maximum impact. Set a budget, communicate your ideas to your florist and ask for suggestions on flower varieties and styles of arrangements. Clip pictures from magazines for ideas.
Most importantly, get your florist involved from the very beginning. You might want to ask him or her to come and look at where you plan to have your party - he or she can give expert advice and creative ideas on how to make the most of your budget and your decorations.