Sa Ta Na Ma
One popular mantra chanted as part of the Kundalini tradition is the phrase “sa-ta-na-ma”. You can repeat these words for the set period of time silently to yourself. Some people like to touch their thumb to each of the other four fingers as each syllable is enunciated (thumb to index finger when you silently say "sa" to yourself, thumb to middle finger when you chant "ta" and so on). You could also imagine a beam of light entering the top of your head and arcing through your brain and then shining out the center of your forehead as the chanting continues over and over. Finally, you could imagine the meaning of each word as it is chanted. Sa means birth. Ta/life. Na/death. Ma/rebirth. You can do all of these in any combination or all together.
Nam Myoho Renge Kyo
Another chant, popular in the Japanese Nichiren Buddhist tradition, is “Nam myoho renge kyo.” As you repeat this phrase over and over for the duration of your meditation, you can imagine the meaning of the phrase becoming your reality. Nam means taking refuge. Myoho/pure dharma or wonderful law. Renge/lotus flower. Kyo/sutra.
The Christian tradition is filled with wonderful chants. It can be a serious Latin Catholic one like “Ave Maria” (Hail Mary) or “Ubi caritas et amor, deus ibi est” (“Where charity and love are, there is God”, or something as simple as “Jesus Christ, Christ Jesus” or “Mother Mary Loves Me, I love Mother Mary.”
The timeless classic of course is the traditional Muslim testimony (Shahada): "There is no God but God and Mohammed is the messenger of God."
Each spiritual tradition, from the major religions of today, to more ancient ones such as the "spells" found in the Egyptian Book of the Dead, the Transcendental Meditation movement from India, or the techniques found in the Hawaiian Huna tradition, has their chants. What the exact wording is generally doesn’t matter.
Jut remember that it is the frequent and repeated practice flowing from the intention you set and follow that moves you towards mastery.
Chanting has a long history across many cultures and spiritual traditions. They key isn’t really what you say but rather that you set your intention to chant a specific phrase for a specific period of time and that you chant that phrase for that period. It could be 10 minutes or two hours or whatever feels right for you (though best to start off with a shorter period and practice it often—frequency is much more effective than duration in making progress.) With chanting, like the other techniques, you are demonstrating your mastery so the content is less important than the general practice. A few examples are listed below.