It's definitely a little scary for some people to stand in front of a large crowd and give a speech, especially one as important as this. During those times of nervousness, it's easy to fall into the trap of attempting to be someone you're not in order to calm your nerves. If you're shy, you might think it's much easier to give a speech if you pretend to be outgoing. If you aren't that funny, you might try to be really humorous to take the edge off. However, the best thing you can do for your speech is to just be yourself.
The bride chose you to be her maid of honor because she cares about you! She and everyone else want to hear what you have to say because your opinion and influence is obviously important to the happy couple. The more you're yourself, the more genuine and honest your speech will be. After all, isn't a genuine speech the best kind of speech -- no matter what your personality?
Just relax and know that you were chosen to be the maid of honor for a very good reason, and no matter what you have to say, the bride and groom will love it because it will come from you!