When to breathe when doing a presentation?

When to breathe when doing a presentation? I tend to talk really fast when presenting and it's really ugly when I have to take a massive breath. It doesn't help if I talk slow either, because I find myself needing to hold my breath for longer. It's also more difficult when I have to project my voice as I tend to speak loud at the beginning and then run out of volume? Does anyone have good resources on this, would be much appreciated!