The traditional posture for meditation in the East is the lotus posture. However, it can be hard to maintain this position. Meditation does not go well with cramp! If you have issues with your knees like I do, it will be difficult at first to maintain that position.
Fortunately, you can meditate just as well sitting on a chair, as long as it is firm and upright. Take care before you start that your spine is erect. Your whole body should be comfortable: poised but relaxed, your feet firmly on the floor, hands resting lightly on your knees or in your lap.
You do not need to wear any special clothing. Once again, the first rule is comfort: don't wear anything too tight. The idea is to forget about the body so that the mind can be free.
Make sure that the room is suitably lit: dim and restful, but not too dark. The temperature should be pleasantly warm, but not overly hot. All these preparations will help you to adopt the correct mental attitude -- and make it less likely that you will fall asleep.
When you first start to meditate, it can be hard to ignore outside noise. You will be in a highly sensitive state in which loud noises are interruptions jar the nerves. So make the family promise not to disturb you, disconnect the telephone, then shut the door. You are now ready to begin your adventure of self-discovery.
Semi Lotus: If you can't quite manage the lotus position, try sitting cross-legged on the floor. If you don't have carpet or rugs down, sit on a thin pillow.
Back Traight: If you use a chair for meditation, choose one with a straight back and spend a few moments before you meditate getting your spine upright.