When you are preparing for your presentation, you should always utilize a script. The beginning and end of your presentation, particularly, must be clear and concise so that there is a clear purpose behind the reasoning of this webinar. You should completely script your introduction and conclusion. The rest of your presentation, however, should have a more conversational tone, especially if you are hosting a live event.
In order to stay on topic, make a bullet-pointed list of the topics that you need to address. This list will help you stay on track with your talking points and images. Since you will rehearse the presentation, there is little chance that you will wander off-topic or lose track of your slides.