First, let's sit down. There are different sitting poses you can choose from and you can find information about them by clicking on the following:
The most important thing to remember is to keep the spine straight and to remain alert. All of the positions mentioned in the above link are intended to suit your needs. If you have an injury or you are lacking flexibility at the hips or knees, choose the appropriate sitting position for you. Keep in mind that you will be holding the same position for longer periods of time and what works for you now may be uncomfortable in 10 minutes.
Some people think that you need to be in "full lotus" pose to be able to meditate. The "lotus pose" is a symbolic pose for a Yogi, because it represents the lotus flower. However, there is no need to try the hardest pose. In fact, in "lotus pose", it is more challenging to keep the locks. Most long-term meditators use the "easy pose".
It's not recommended that you lie down on the floor in Savasana or corpse pose because these poses make it harder to stay alert, and the mind will soon drift away. That's why it is called "corpse pose", because it simulates a dead body.
You can also sit down against a wall, but this should be done only at the beginning. It's not advisable that you depend on other objects.
Chair pose is always a good choice for people with problems in the joints. Choose a good firm chair to avoid curving the spine. Keep the legs uncrossed and the soles of the feet on the floor.
Cushions can be a great help because they place the hips higher, relieving tension on the knees. Different paths use different types of mats and cushions. In general, the surface should be firm, not too firm but not too soft either.
The less you move the better concentration you will achieve. In the beginning, you should start with short periods of time like 3 minutes, 7 minutes, 11 minutes, and then build up from there. There is no point in hurting yourself. Meditation must remain as a pleasant act.