This is the very first meditation technique I used when I started meditation. A Marta is a saying or a specific word that you repeatedly say to yourself to distract your self from other thoughts. I remember my first ever Marta I used was from Rebekah Bonrucki from Bexlife.com. The Mantra said "I am like a boat on a ocean, my thoughts come in and like waves. They rock me just a bit but they never carry me away. I am anchored in the hear and now.
This specific Mantra really help me when I was still living with my parents and they were not particularly getting along which would often. I would sit in my room and repeat this Mantra in my mind until thing's calmed down.
One of my favorite Mantra's I use today is the Gayatri Mantra in English translation means'We meditate of the glory of the creator. Who has created the universe. Who is worthy of worship. Who is embodiment of knowledge and light. Who is the remover of all Sin and Ignorance. May he. enlighten our intellect." This Mantra was written in Sanskrit around 2500 to 3500 years back.
There are literally thousands and thousands of mantras in the world each with their own meaning. Of course you can always create your own mantras to help you with your current situation. Mantras truly are a great technique of meditation.