Take a deep breath. If you are like most people, you will have have taken a short sharp suck in of air with your shoulders lifting to fill your chest. Your lungs will not even be close to full. When you exhale, once again the process will be short and sharp. Learning to breathe properly is one of the best ways to learn to relax. It feels strange that we need to actively learn that as a skill. One of the most difficult Yoga poses, is the corpse pose (savasana). Learning to slowly release the tension from your body, partly by focusing on your breath.
To practice breathing deeply, you need to learn to use your diaphragm. You can lie on your back and loosely interlock your fingers. Instead of your tummy going in as you breathe in, it should go out. This is your diaphragm pushing down and opening up space for your lungs to bring in more air. As you breath in, you should feel your fingers part slightly as your tummy goes up. Fill your abdomen. Then your chest. Then carry on right up to your clavicles.
When you exhale, try make it as slow and long as the inhalation. The more you can slow down both, the softer, the quieter, the more air you will be able to pull into your lungs. The process of focusing on your breath is meditation. The idea of meditation can be overcomplicated. If you are constantly bringing your thoughts back to your breath, even if they wander, you are meditating. Just find somewhere comfortable to sit, or do it while you are walking/running, or lie on your back (without falling asleep).
Along with talking to understanding friends/professionals, or getting medical help, meditation is one of the best ways to improve your mental health.
Start with abdominal breathing.