Perhaps try to speak your thoughts aloud and every time you find your self speaking pull your mind from that thought, quiet your lips, and return to focused meditation. Mindfulness just takes practice. Meditation is that practice but it's not easy and won't come to you quickly necessarily. As long as you are aware of what your mind is doing and where it is going you can return it to your meditation.
I tend to focus on breathing and the flow of prana in my body, trying to concentrate on gratitude when my mind wanders off.
Use guided meditation! Guided meditation helps me focus on my breath instead of my own wandering thoughts. This app has many great options!
Concentrate on one specific thing such as counting, a river flowing, or clouds passing by. When a new thoughts comes along, watch it go down the river or float away on a cloud.