Content is king
Have the words up your sleeve. For a wedding speech, you need to know EXACTLY what you're going to say.
Unless you are a very practised public speaker, it is unlikely that you'll be able to get through an entire wedding speech without having crafted at least some of it beforehand. Trying to give a completely improvised or note-less wedding speech is only for the very brave, or very foolhardy!
Ultimately, your speech will be remembered for what you say, not how you say it, as long as you speak clearly enough for all the audience to hear you. Being able to speak entirely without notes is a skill to be admired in many situations, but it is important to remember that a wedding speech has very specific requirements. It needs to be interesting, meaningful and entertaining, without stepping on any toes or upsetting anyone, which is a fine balance to strike at the best of times. For the great majority of people, that means putting in some pre-speech preparation.
Scripting some entertaining jokes or unexpected punch-lines may feel odd at first, but after you have practised saying the lines out loud a few times it should become more comfortable and flow more easily. In fact, many people unused to public speaking find that having a script to work to makes the event flow more smoothly and calms more nerves than trying to be more off the cuff. A wedding speech also needs to be suitable for all the audience, and the best way to ensure you don't make an inappropriate remark, or a too near-the-knuckle joke, is to make sure you know in advance what you are going to say. You don't need to write it out word for word and stick to it religiously. Figure out what best suits your speaking style, but remember that sometimes you do need to be prepared, and giving a good wedding speech is definitely one of those times.