As you're asking specifically for using powerpoint, I would like to suggest few tips you can use.
Colors:
When choosing colors for presentations, use http://colorhunt.co, http://Coolors.co, or http://colorlovers.com, as you'll find prebuilt color templates that look quite good than the stock colors of powerpoint. This will definitely save you a lot of time on deciding colors.
Fonts:
If you can, install google fonts and use them instead of stock fonts. Most of the stock fonts are overused in the industry (comic sans) and gives your presentations a boring look. You can use http://canva.com/font-combinations for fonts suggestions that go well togather. One for headings and other for content. You can choose a thrid font for comments or captions.
Template:
Here, I would suggest that many other are suggesting. Use your company's provided template. It would be much easier to modify that than to start from scratch and think of something new. Still if you want to use something new, watch a lot of presentations, especially the featured one on Slideshare.
And if Powerpoint is not the only option:
I would highly suggest using http://canva.com for your designing needs. I, from past year, have been using it to design almost everything. From presentations to Infographics and deliverables.
3 benefits I would like to highlight -
1. It is much more flexible than powerpoint to modify, add colors, add pictures, resizing your designs.
2. Provides hundreds of shapes, illustrations, images, few good fonts (for free). This saves a lot of time.
3. Offers multiple free templates to start with.
The major of all is, its free and you can use it anywhere, on home, office. Share the files easily with just a link.
PS: I'm not related to Canva's marketing or anything, I'm just a too happy client of them. I haven't even purchased their premium package as I never felt the need.