Try a guided meditation or visualisation. This can help you to achieve a relaxed state quickly. It is important though, to be able to achieve focus on your own.
Like sopredictable said, focusing on the breath is a good start. Progressive Muscle Relaxation is another good way. You'll probably find places in your body that you are tensing unnessecarily. Just start at the top of your head and focus on all of your body parts down to your toes. Spend 5-10 seconds on each body part and aim to relax it completely.
Stress can make it difficult, especially in the beginning, but it is a difficult thing to do and it does get easier. But even if you have to remember to calm your mind a thousand times, you're making progress.
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Sticks n stones'll break my bones, but if I land the first one, you're in trouble!