Before you speak do your homework.
The essential point of a good speech is making a resarch about the audience. Before you start preparing it, define. Who will be listening you?
How old will be your listeners? By defining avarage old of your listeners you will make your piece more accurate. You need to remember that everything matters. By talking to teenagers you will use different vocabulary than talking to a group of adult people. Also metaphors you use will be not the same.
What are they problems? Knowing their probable problems will help you to make your presentation more powerful. You can use a piece of your time to show them possible solutions, give them your adivice or to explain them reasons of those consequences and tell how they can change them.
What they want to achieve? This questions is one of the most important to ask. Thanks to it, you will see the purpose, the goal of your speech. Constantly ask yourself: how can I help them? Looking from my experience, what will help them the most in achieving their goals? Knowing what you now know, what tips would you give them to avoid making bad decisions?
While delivering your speech, have the image of the purpose. What do I want to accomplish by saying this? What do I want to give and share with people?
Education. Not only age matters. Knowing from which field, how advanced your listeners are will let you know what kind of vocabulary you can use. Using professional vocabulary may cause that laymen will not understand your message.
Remember: while using more advanced words, always explain them to your interlocutor or audience.