Be comfortable
Unless you are physically comfortable, there is no way you can meditate! Meditation requires you to forget the body. If the body is in pain, how are you going to forget it and move beyond? So ensure that you are comfortable.
Many techniques require you to squat on the floor. If you are not accustomed to this, use a chair – but keep trying. Use a cushion as a seat if you wish to. For lie-down techniques, don't lie on the bare floor. Use a carpet or a straw mat.
Ensure that some time has passed since your last meal - at least two hours.
Initially, you may feel sleepy during meditation. This is the tamas (lethargy) that is weighing down your system. Don't try to fight the sleep – ultimately, you will be able to neither sleep nor meditate! And don't feel guilty about feeling sleepy. If you are sleepy, just go to sleep. When you finish sleeping, you can get up and meditate, that's all!