Meditation is not an exact science meditate there are many ways that you can meditate and there are many things that you can do when you want to. Why is the first question that pops into your mind I presume. Well this is because you need to go out and find the rhythm and the pace that you need when meditating. For beginners, it is advisable to start slowly and then build up to longer time spans.

Posture is relative.
Many people stress that you need to have a good posture if you want to have a nice time in meditation. Many people believe that posture is important because a bad posture may lead you to focus your discomfort and may cause you to lose focus. However, the main point of meditation is that you need to be comfortable in the way that you are seated when you are meditating. It does not matter if you have good or bad posture but it matters that you are comfortable.

Breathe through your nose and not your mouth.
Deep breathes from the nose are more relaxing than breathing from the mouth. Keep your mouth shut at all times in order to keep your body in a relaxed state.

Relax.
If you feel or are suddenly conscious of a certain body part then it is a clear that this part is still tense and you need to focus and make this part relax.