Meditation can be as easy as breathing. You don't need to sit cross-legged, or be surrounded by complete silence. When you feel your stress levels rising, simply focus on your breathing — it will help bring your heart rate down. If you can, close your eyes, but it's not essential.
Breathe slowly and deeply through your nose into your belly. If it helps, you can try a breathing technique that's said to aid sleep: breathe in for four, hold for seven, and release through your mouth for eight. If your mind wanders, acknowledge the thoughts, let them pass like clouds in the sky, and gently bring your attention back to your breath.