Much attention should be paid to spinal alignment, at the beginning, and during, your meditation sessions. In some Yoga classes, you may be instructed to sit in Padmasana (lotus posture). As you will learn, this is not entirely correct for every student, but the reasons for choosing Padmasana are logica Ardha Baddha Padmasana benefits.
With the legs crossed, they serve as a strong base, which keep the spine straight, so that energy will flow throughout the body; especially up and down the spine. Yet, this posture is not good for everyone because of the stress put on the knees. In some cases, forcing students to sit in the lotus posture will stress the ligaments within the knee joint.
In the case of knees that do not conform to lotus posture, half lotus (Ardha Padmasana) is often chosen as a secondary posture. Sukasana (easy posture) is also a stable choice for meditation. Modifications can be made by placing blankets under the knees or sitting on the blanket.
When I first began to meditate, we learned to sit in Vajrasana (thunderbolt posture), which is also called, "Seiza" (kneeling posture), in many Japanese and Okinawan martial arts schools. The kneeling posture also keeps the spine very straight, and this posture can be modified by sitting on a bolster or a rolled blanket, which runs between the legs and under the pelvis.
There is also another common modification - by placing a blanket or pillows between the calves and leg biceps. Again, the purpose is to take stress off the knees, which makes it easier for a practitioner to sit in meditation a bit longer.
As you can see, there are many choices, but the primary objective is to sit with the spine in a straight position. Some people choose to sit on a "Zafu," which is a round cushion. Traditionally, a Zafu was black, about 14 inches in diameter, and 8 inches thick. These days, you can find them in a multitude of colors and ergonomic designs.
This leads into the idea of meditation props. There are a variety of meditation chairs, pillows, benches, cushions, and Salubrion Seats to choose from. In fact, you don't have to sit at all. Some people practice meditation from a standing position, and there is also the option to do a walking meditation.
At all times, the spine is held straight. When you think about all of the above-mentioned options for meditation, choose a posture, which you can hold without discomfort. If you want to stay with your meditation practice, comfort is part of the equation. People who feel they must suffer through a posture will not continue to expand their meditation practice.