There are thousands of mantras out there. What do you want in your life right now? A new job? Love? Motivation? Choose a mantra and put those desires out in the universe. You'll be pleasantly surprised with the results!
When I started doing yoga, I loved most of what happened in the classes. One thing I did not love, though, was the chanting of "om." None of my early teachers could explain what it meant or why we were doing it.