The bridal bouquets in weddings are a tradition from the time of royalty. Bouquets can be tied 3 different ways, these ways are nosegay, hand-tied, and cascading bouquets. Another tradition is that once the bride is married she throws the bouquet over her shoulder and the person that catches it is the next person supposedly to get married. The origins of this tradition is like I said before was from the time of royalty. The rich people of that era were the only ones able to afford bathtubs, even though they never really used them because bathing wasn't part of the routine for people of that time. But during the wedding the royal bride that was walking down the aisle she held the bouquet of roses or other fragrant flower so the the body odor would not spread.
The most common way to get a bouquet is to buy them from a florist, you can pick what you want in it. How big you want it, and when you want it. Before the wedding all you have to do is pick it up, but some people want to make their own so they can have a one of a kind bouquet. If you are wanting to make your own there are many websites that give you directions on how to tie them so they don't fall apart during the wedding.

One thing that some people also do is to have not only the bride hold a bouquet but also all the bridesmaids. These are usually different from the one that the bride is holding. They typically use the same type of flowers but use different colors they are also normally smaller, and have a different pattern, that way they don't take away from the brides bouquet. This is totally optional though, and don't have to be done if the bride don't think it would work.

Here are some pictures of bridal bouquets that you may find interesting, and hopefully they will give you ideas on what you would want on yours. As you will see you can use any type, color, or size flower you want. Just make sure it looks structured, and not just thrown together.
Bridal bouquet with white Calla Lilies, and a burgundy accent flower.
Cascading bridal bouquet with Roses, Lilies, and colorful accent flowers.
Hand tied bridal bouquet.