Many people have trouble relaxing and concentrating when they first begin meditating. The trick is to begin with very short periods of meditation, say five minutes. As you improve, gradually lengthen the time, but only by a few minues at a time. Don't jump from five minutes to half an hour in one session. And don't give up. Meditation is a skill that can be learned just like any other if you keep working at it.
Its also helpful to find a mantra, and begin meditation by chanting. This gives you somthing to concentrate upon and is very relaxing. It should be something short and simple and easy to remember.