It is important to assume the correct posture while meditating. There are two things to consider when doing this:

1) The spine should be erect. Slumping of the shoulders, or bending of the spine, prevents the energy currents in the body from flowing freely. If this happens then we create energy blocks in the body. Think of energy flowing up and down your spine like a water would move through a pipe. Not only that, we are unable to experience the fullness of breath that is required if we are slumped forward.

2) Our posture should be comfortable so that we can sit in it for the intended duration. For most beginners that means sitting on a chair. It cannot be a chair with a back though. Leaning against the chair back will put pressure on your spine and it becomes a distraction from meditation. Choose a flat chair with no back to it, or at least keep your back off it. In this seated position you would want to have thighs parallel to the floor and feet flat. You can rest your hands on your knees with your palms upturned. Your chin also wants to be parallel.

You may feel comfortable sitting on the floor on a cushion, especially if it is only for a short time. You can sit cross legged and remember to keep your spine upright, the same as if seated on a chair.

If you feel particularly nimble you can even assume the posture shown below, known as the Lotus Posture. Here you can see that you would rest your foot on the opposite thigh. This does require some dexterity, maybe work your way up to it!