Please see the about section for suggestions on different meditation positions. (https://thebreathnow.wixsite.com/website/about-mindfulness)
How do you know what position to pick? It depends on how you are feeling in the moment. Most people like sitting on the ground on a cushion. However, if you have been sitting all day, then you may want to stand. If you need to lie down, then feel free to do so and try to stay awake during the practice.
At the end of the day, no matter what position you choose, if you are comfortable, then longer you will be able to practice in stillness!