Often when people are just curious, there is some deeper underlying question. If the question is big enough and intense enough, you won't be able to answer it. You'll just sit there not knowing the answer. Let go of your thinking, opinions, and desires and continually return to the questioning mind. In zen meditation, mantras are used to calm the mind and strengthen the center.
Any mantra is fine.
There are many, many others. The usual technique is to recite the mantra constantly, paying attention to it and allowing all other thinking to drop away. If your mind wanders, gently bring it back to the mantra and keep on going. There was a seal on old public documents that were copied. One half of the seal was on the copy, and the other half on the original.