One we all are familiar with is the often-satirized "Om" meditation. Either chanted aloud or mentally generated, one focuses on the syllable "om" or "aum," which is the sound of the universe according to Tibetan tradition.
Other cultures prefer something a little less abstract. Some prefer simple phrases such as "We are One," or "I am patient/loving/compassionate," or even one word: "Peace." Whatever you choose, make sure it reflects something that you feel that you need to focus on in your life at that time. What you focus on, you become.
(photo courtesy of pixabay.com)