Toasting the happy couple at the wedding reception is one of the event's most cherished traditions. Offering a toast is said to have started in sixteenth-century France, where it was customary for men to toast the health of any women present at a banquet; it soon evolved into toasting the bride at the wedding feast.
Traditionally, the first toast at the wedding reception is offered by the best man. The toast is usually offered once everyone has been seated for dinner (or in the case of a cocktails-only reception, once the receiving line is completed and everyone has been served a drink). If grace will be said before the meal, the blessing precedes the toast. When everyone has been seated, the best man may take the microphone and offer his toast. When ready, he attracts the attention of the guests -- it's a good idea to use a microphone if one is available so that everyone can hear -- and proposes a toast to the bride and groom.
The bride and groom can certainly make a toast together, or both of you can make individual toasts. Your toast can be offered at any time during the reception, as long as it's after the best man has made his toast. You and your husband should stand together and take turns speaking. As for what to say, that's entirely up to you. Speak from the heart, offer your appreciation and thanks, but keep it short. A wedding reception isn't the time for long anecdotes or jokes; save those for the rehearsal dinner or bachelor (and bachelorette) festivities. Wedding reception toasts should be short and sweet.