Standing yoga poses can often be quite challenging, especially for beginners who are not very flexible. A good way to start is to use a set of yoga blocks under your hands to help you achieve correct alignment and to hold the poses longer.
Stand upright with your feet together. On an exhalation, bend forward at your hips and place each hand on a yoga block. Make sure each block is directly below the shoulder. Shift your body weight slightly forward so you are balancing over the middle of your feet. This way, your hips can line up directly above your ankles. Pause for a moment, lengthening your chest to maintain a straight, long spine.
Shift your weight over your right leg and slowly raise your left leg behind you to hip height. Look slightly forward and lengthen your chest away from your hips. Be sure to keep both legs straight, with your left knee and toes pointing down. Think of pushing out with your left heel. Hold for three to six breaths, then lower your leg, switch sides and repeat. (As you get more flexible you can practice with your fingertips on the floor instead of using the blocks.)