You may have decided to take up meditation but not be sure how you should sit. Of course some people lie down to meditate but there's a danger of falling asleep!
Also you can meditate while walking but the general practice is to sit. So what should I sit on - a chair, cushions, a stool or cross legged in a lotus position? It's not a bad idea to start by sitting on a chair but if you want to develop your practice you may want to move on to a sitting arrangement that you specifically associate with meditation.
Whatever you decide, there are some essentials: you should be comfortable - aches and pains will distract you; your spine should be as upright as your body will allow - not in a rigid sargeant-major way, but in an upward energy way, you want to be relaxed but alert; to help to keep your spine upright your knees should be lower (or at least not higher) than your hips. Meditation guides will give detailed advice on achieving a good posture and a good teacher or other meditators can help with advice.
In my personal experience it's taken years to become able to sit comfortably, that's partly due to trying to be too ambitious. Nowadays I sit on a chair or, preferably, on a folding meditation stool. If you want to learn to meditate but don't know where to start, this is a fantastic 10 minute meditation App.
It's a guided meditation which makes it great for beginners but it can also revitalise the practice for experienced meditators. Andy's voice and delivery is spot on and the helpful graphics and commentary are delightful and clear. http://www.getsomeheadspace.com/.
If you want to move on from sitting in a chair, here's a couple of meditation stools that are tried and tested - I use the folding stool every day; the mushroom stool packs up smaller and is good for experienced meditators and for when you're travelling light and going on retreat.