1. Meditation helps you to learn how to focus better. It's kind of like a mental training excercise. In fact, studies suggest it also has an effect on cognitive abilities. You can use meditation to calm yourself, get yourself in the right frame of mind and can help you mentally prepare properly for casting.
2. There are lots of different forms of meditation found in many cultures. Each type of meditation and each culture obviously has a different history. The earliest record of meditation comes from Vedantism (a branch of Hinduism) and dates back to 1500 BC. However, evidence suggests that many cultures developed meditation independantly of each other so giving a complete history would be quite difficult.
3. A lot of people suggest the typical crossed legs position. Others like to kneel or to sit in a comfy chair. I would say whatever makes you feel comfortable or whatever feels right to you is okay. Just don't get too comfortable or it'll be hard to stay awake if you meditate in the morning or at night.