Get a pen and two pieces of paper.
Page 1 is where you map out your story. Think about your audience: what do they need to know?
Split the page into three sections- a beginning where you tell them what you're going to tell them. A middle where you tell them. And an end where you tell them what you've told them.
Fill the sections in per above but think carefully about how to make your content engaging and memorable. I usually use stories that I tell while showing pictures then have summary slides that have words on them. Try to avoid bulleted lists.
Then use page two to map out what you want each slide to look like.
Then you just have to make the slides. You can use the embedded templates in .ppt or there are loads on the web. Nick a design from someone whose PowerPoint is good and then make changes to it.
General style guide:
Avoid too many bullets
If you want people to listen and not read then use animation to control what they can see at any one time (but keep it simple, nothing needs to bounce or twist) but you'll need a clicker when you present it.
Use consistent colours throughout - less is more.
Use "align" and "distribute" commands to get everything to line up, either with the edge of the slide it with other elements.
Remember that the content and story are the purpose of the exercise.
Good luck.