Watching the breath was my first practice in meditation. I often instruct people to put their attention on the inhale and exhale coming from their nostrils. No mouth breathing in this meditation.
Slowly, this evolves into putting the attention on the breath during the day at work, at home, outside, working out, practicing yoga, etc. This is mindfulness in action. When I return my attention to my breath, I let go of thoughts about the past and future. I am here, now.
Learning how to breath is another important step. When practicing pranayama (breathing and life force) exercises before meditation, make sure you begin with a 3 part yogic breathe, which can be seen here. This will help to breath deeper and bring more oxygen into the blood.
I would recommend finding a consistent spot for your meditation practice and meditate at the same time everyday. For many of us, this will mean waking up earlier. It's worth it. Besides reducing stress, ADHD symptoms, nervousness, and tension, meditation rebuilds gray matter in the frontal cortex.
Mindfulness begins with a practice in meditation. Bringing meditation into your everyday life is the action of mindfulness.