Of all the components of a wedding, flowers are one of the most important. If for no other reason than the fact that flowers tend to be in massive abundance at a wedding. From the table centerpieces to the bridal bouquet, flowers play a large part in nearly every wedding ceremony.
With regard to selecting the flowers for your wedding, a number of factors must be taken into consideration. First, obviously, the individual preferences of the bride and groom are important. If a particular type of flower is adored, or hated, by either individual then it's probably important to take that into consideration.
Next, you need to consider the color coordination. There is a color scheme to the bride's dress and the dresses for the bride maid's. While not everyone will notice, if the flowers stand in start contrast to color scheme of the dresses, it will prove to be an eyesore. Plan your color scheme and then begin flower hunting for complimentary colors.
One of the factors regarding your floral selection which remains, relatively, outside of your control is the seasonal aspect of flowers. If you have your heart absolutely set on a particular flower then it's obviously possible for you to plan your wedding for a time when they are in season. Most people, however, pick their wedding date and then start trying to figure out their floral options.
If you have a very specific idea regarding what you want your floral arrangements to look like, take care to ensure that this is communicated to the florist. It's probably worth drawing them a picture even if you have the art skills of a 2nd grader. In other words, do everything you possibly can to ensure that you and the florist are on the same page. Many weddings have been rained on at the last minute when the image of the flowers and the actual flowers bear only a passing similarity.
If you don't have an image in mind, you're probably better off giving the florist a budget and letting them use their experience and creativity freely. As long as you're using a reputable florist, you can count on the fact that they didn't get a good reputation by creating poor ensembles. Working within a budge, a florist can use the least expensive attractive flowers available, which is information you're not necessarily privy to.
For any large floral piece, such as those seen on an altar, keep in mind that most people will only be seeing them from a distance. These arrangements present an excellent opportunity to use large and inexpensive flower types.
The odds are that you're not a flower expert. That's why you hire a florist in the first place. Talk with them, listen to them and give them some freedom, within your budget, to create something unique.