Do I need to sit in the Lotus Posture?

No. The posture you choose for your meditation practice should allow you to remain both alert and relaxed, but that can be achieved sitting in a chair just as well as sitting with crossed legs on a cushion. It is even possible to practice while lying down, but the risk of falling asleep is great and so it is not the best choice for most beginners unless they have physical problems that prevent sitting upright.