Do I need a teacher?

It is theoretically possible to learn meditation from a book. However most people who teach and practice meditation agree that a teacher can be an invaluable aid in learning a meditation technique and making sure it is practiced correctly. The beginner will usually have several questions, which a teacher will be able to answer. Also, learning with a group of people, e.g. a meditation class, allows you to experience the benefit of meditating with a group of people. Most people find that they have some of their best meditations while meditating in a group, because there is a collective energy and focus present.

Various individuals and groups teach meditation. Some charge and some do not. Many different techniques are taught, some more spiritual in nature and others mainly concerned with stress–reduction and gaining a little peace of mind. As always, the important thing is finding what works for you.

How is meditation different from relaxation, thinking, concentration or self–hypnosis?

Relaxation: Relaxation is a common byproduct of meditation. Relaxation itself can assume many forms, such as taking a hot bath or reclining in the Lazy–boy and watching TV, etc. Meditation is an active process where the meditator remains fully aware of what the awareness is doing. It also attempts to transcend the thought process whereas many forms of relaxation still engage the thought process. Meditation allows the body to relax and can offset the effects of stress both mentally and physically to a potentially much greater degree than passive relaxation.
Thinking: Thoughts generally consume energy in the process of their formation. Constant thought–activity, especially of random nature, can tire the mind and even bring on headache. Meditation attempts to transcend this crude level of thought activity. Through regular practice one becomes aware that they are not their thoughts but that there is an awareness that exists independent of thought. Descartes (I think, therefore I am) obviously was not a regular meditator!
Concentration: Meditation begins with concentration, but after an initial period of concentration, thought activity decreases and keeping the awareness focused becomes more spontaneous. At this point the person may or may not continue to employ the object of concentration.
Self–hypnosis: Self–hypnosis, like meditation, involves at least an initial period of concentration on an object. However in hypnosis one does not try to maintain an awareness of the here and now, or to stay conscious of the process. Instead one essentially enters a sort of semi–conscious trance.