One of the most useful things to learn within PowerPoint is about using the Master Slide. This is a way of formatting the entire presentation in one place rather than applying formatting on each individual slide. This will save you time, improve the quality of the presentation (as formatting will be consistent throughout) and will make the presentation much more accessible, as it then becomes very easy to change the colour scheme for a presentation if a learner requires it.
It is very easy to do, and these 2 short screencasts will show how to set up a basic master slide, and then how to format the header and footer options to include things like slide numbers.
To create a basic master slide
To then add a footer such as slide numbers