Hello Suttida.
I am sorry about your loss. May his soul rest in peace. The center of the body is in the middle of our abdomen two fingers above the navel. At this point, we bring our mind during meditation. When you meditate, you need to let go of thoughts and easily access your center without struggle.
Try to center mind if running errands during the day. Thes will make is easy for you to access your center during meditation.
All the best
Tim