Firstly, the time to start your preparation is as soon as you know about it. For example, if you’re being asked to present in three weeks’ time, it’s tempting to put off the preparation until a week before, or even later.
Much better to get on with the collection and preparation of your material as soon as you know you’ll be speaking. Keep a notebook handy and become a collector of ideas, as they occur to you. Or use an app like Evernote.
Early on create a Talk Map. How should I prepare my presentation? This will give you ideas on what to include and therefore what information and ideas you’ll need to collect. If you get stuck for ideas ask someone else you trust to help you out. Even if they don’t know the topic, their thoughts and suggestions will probably help you to get you moving.
For many presentations, material that you can use for content is all around you, but you probably won’t see that at first. Just think of some of the good presenters that you have heard. They will speak about everyday occurrences and situations, even in a technical talk.