The slides you include in your presentation should support an overall narrative or "story." As such, you should define your story prior to making slides. What is the problem? Why is it important? How am I approaching it? What results do I have so far? What have we learned? etc.
It is often difficult to articulate a narrative after months at the lab bench and many scientists begin the presentation process by creating slides. Slide making can get you thinking. That's OK, as long as you step back at some point and articulate your story in some type of narrative form. Otherwise you will end up with a largely meaningless collection of slides. The slides-narrative process can be iterative.
After defining the narrative, the process of slide making may lead you off in unanticipated (by the narrative) directions. Perhaps you remember some really exciting data that doesn't fit with the narrative. Fine, include it, but pull back and rewrite your narrative. A coherent narrative is the essential building block of a presentation.