If you've started practicing yoga recently or are still thinking about your first steps into the world of yoga, keep in mind that yoga is not a competitive form of exercise. You have all the time needed to practice all the asanas and postures you want. Take it easy and start with relatively easy postures or asanas before moving on to harder positions.
The best thing about yoga is that you can modify a posture to suit your comfort level. Of course you are not going to change it completely, but you can shorten or extend the duration of the posture according to your body's requirements. Simple variations and props also help to make yoga postures easier for beginners.
The first and easiest of yoga poses is the Mountain Position (Tada Asana). This position is all about standing in a proper way. Stand with your feet touching each other and your shoulders, head, and pelvis in the same vertical line. Hold this position according to your own convenience.
From the Mountain Position, you can get down to the Downward Facing Dog Position (Adho Mukha Svana Asana). This position is very popular. To perform this exercise, lie on the ground with your arms underneath your shoulders and your knees underneath your pelvis. Push up until you form an inverted "v" shape with your arms, head, and back forming one side of the V and your extended legs, the other side.
After these two positions you may want to proceed to the Deep Yoga Squat Pose (Mala Asana, or the Garland Position). This is a complete squat. Hold this position for anywhere between half to three minutes. This workout is very good for your pelvis, back, and groins. It is also called the pelvis opener.
If you feel that your body's flexibility has developed a little bit, then you may want to proceed with the Warrior II pose. Position your right foot forward with your thigh parallel to the ground. Keep your left leg stretched backwards, extend your two arms and gaze forward and tuck your belly in a little. This exercise is directed towards the calves, arms, legs, and back.
After you practice these positions, ask your yoga instructor and he will guide you to more advanced techniques and postures. Do not forget to breathe in the proper way while you are doing your positions. Good breath will help you maintain your positions for a longer time; it will also relieve you of any exhaustion.