Om is the simplest of mantras – and easy to learn. Some schools of thought decree an exact pronuncation, but I have never subscribed to this pedantic, narrowing view. There are not too many ways you could possibly pronounce it anyway. And Indian researchers, who have been studying Om, have found that it doesn't matter how you say it, how fast, or slow, it always has an essential 'Om-ness' to it. And it is this that meditators focus on when chanting the mantra.
We inhabit a world of thoughts, perceptions and intentions. Just consider that for a moment. Mantras are 'thought vehicles' in a way, verbal (or internally expressed) representations of thoughts or intentions. Thinking of what 'Om' symbolises, It is a sacred, powerful sound. And when you begin to chant it, timing it with your breathing, perhaps you begin to understand how you are the breath and the breath is you; you are the Universe and the Universe is you. All of it carried on a sound...
This simple mantra and 17 more are in my latest offering. More info on this page.