The wedding dress usually dictates the flower colour theme so, if possible, give your florist a swatch of the dress fabric and a photograph. It's also important that the florist sees the style of your head-dress and the bridesmaids dresses.
The type and style of flowers you choose for your big day are personal to you. You should bear in mind that you may have to organise all or just some of the following:
both your own and the bridesmaids' bouquets
flowers for any head-dresses
baskets or tied bunches for the flower girls to carry
button holes for male guests
corsages for female guests
flowers for the ceremony venue
flowers for the reception venue
table and top table centre-pieces
If you can, arrange for your florist to visit both your ceremony and reception venue prior to the wedding (or show them pictures) so they can get an idea of scale.
Check that your florist can arrange delivery of flowers to all relevant venues and, if need be, set them in place. Make sure she/he has all relevant contact names and addresses in advance.
Remember that if certain flowers are out of season they may be significantly more expensive.
If you want to have your bridal bouquet preserved, you may want to order a second bouquet for throwing to your guests.