If you learn the yoga in class, you will get better result than learning by yourself, however, if you cannot attain the class, you can buy the books or DVD online to learn by yourself. If you learn by yourself, you have to select the easy postures, as some postures have to be supervised and watched by the yoga teachers and you have to read the rules of practicing yoga. The following link is the example of the yoga rules:-
The following free streaming videos are from www.youtube. In yoga class, the yoga teachers normally start with the yoga warm-ups and then follow by the 'surya namaskar' posture (video no. 5) before teaching the new postures, as the surya namaskar posture is the posture to exercise all parts of the body.