Well, because the mind tends to be more alert and attentive when we're seated and upright, most teachers agree that sitting to meditate is best whenever possible. However, if you're wondering whether you can meditate lying down, the answer is yes.
Posture is essential to meditation, but you can take a flexible approach to it. Start your practice while in a position that comes naturally to you. It's important to start in a comfortable place, so that you can gently shift your body into the correct positioning throughout your practice.