Many of the best wedding floral designers are booked well in advance. Begin by gathering your ideas. Of course the internet can be a great inspiration for ideas as well. Try looking at wallpaper or fabric books. They have wonderful ideas for color palettes.
Next step is to ask for recommendations from family and friends.
Then start shopping around for floral designers. Believe it or not, at this point you do not have know the color of your bridesmaids' gowns, nor do you have to have a color scheme in mind. Your concern is to view their work and to feel comfortable with the the designer. Don't be afraid to ask how many weddings they've done and to see photos of their work. Once you have narrowed down your choices for floral designers, secure the date by leaving a deposit.