A mantra is a phrase or a sound you can repeat during meditation to keep your concentration, so if your mind wanders you could use "Om" which I read means something like "it is" or "to become" and carries the vibration of an intention manifesting into the physical realm. Or you can use something like "I change my thoughts, I changed my world". 

Affirmations are more general things which support an intention, more like emotional support and guidance related to whatever it is you desire. For example if it's confidence you seek you can use "I am confident", "I radiate confidence", although from experience I am and I have are the most effective affirmations to use.