You can breathe many ways while meditating. Noticing your natural rhythm is one way. You could practice deep breathing where you count 2 or 3 seconds in and 2 or 3 seconds out. Many times instructors will have you practice breathing into your abdomen. But I think noticing your breathe is a good place to start.
Breath in through you nose and take a big breath out from you mouth letting your self deflate and all your worries go out with a clear mind
Slow and steady. In through the nose, out through the mouth. Breathe deep from your diaphragm and focus on your breathing.
It depends on your goal. Simple mindfulness meditation usually asks you to breathe as you normally would and just pay attention, while some others may ask you to slow your breathing, usually with counting.