Read The Joy of Living: Unlocking the Secret and Science of Happiness by Yongey Mingyur Rinpoche. It will answer all your questions about meditation, how and why it works, what method is the best for you, etc. You can buy it here.
If you are too tired for sitting in a meditative pose, you can lay on your back and meditate, just keep your back straight and relax your body. Most of the meditation methods will work so.
In some days of such practice, you will feel more energy and you will be able to sit in a usual meditative pose.
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