You have limited time, you have limited budget, and you are not a natural-born designer. Still you can design an effective presentation. Keep it minimal and tasteful.
  1. No bullet points
  2. One big idea per slide
  3. Use simple shapes without a border
  4. Muted color palette (lots of greys, one bright accent color) without gradients
  5. Non-cheesy stock images: high res and in correct aspect ratio
  6. Arial font
  7. White page, (small logo if you want at the bottom right)
People will recognise it was not designed by a professional, but they will get the message and respect you how you did this with such minimal tools.