Yoga is generally seen as a low-impact, low-stress activity. However, injuries do occur, and you will want to prevent them. Do not be too worried, the injury rate reported by yogis is significantly lower than other sports, but there are some common injuries that are quite avoidable.
Remember, You Are a Rookie
The most common cause of yoga injuries is pushing yourself too hard. While challenging yourself is going to help you grow in your practice, taking this too far is a great way to hurt yourself. Stick to classes that are within your experience range, and stop if you feel uncomfortable. Asking your instructor for a modification is also a great way to stay involved in a more advanced pose, while not taking any risks to your personal safety. Props are also there to modify poses to make them more achievable for a new yogi.
Bend Those Knees
You can hurt your lower back in a number of different poses. Forward Folds is a pose where you can be susceptible to rounding your back to get further into the stretch. Keep a little bend in your knees, and do not try to push the stretch all the way to the ground if your flexibility is not quite there yet. It is better to set your hands on your ankles than on the floor if it means you do not pull muscles in your back. It is also possible to pull your hamstrings in a Forward Fold when you push the stretch too much. Being a little uncomfortable in a stretch is fine, but it should never hurt. Pain is a sign that your body is not ready for what you are trying to make it do.
New yogis are likely to experience a little bit of pain in their wrists. Many different poses, such as Downward Dog and Plank, are going to put more strain on your wrists than you are used to. This is going to be more pronounced if you already have weakened wrist due to carpal tunnel syndrome or too much time spent on a keyboard. To reduce this, make sure that you are putting your weight on the correct part of your hand, and spreading the force out over all of your fingers. The angle between your hand and forearm matters a lot too. Pay attention to your alignment during these poses, and do not be afraid to ask your instructor to help you out on getting it just right. As you practice, you will find that your wrists are able to handle more as they get stronger.
On the odd occasion that a yoga injury does occur, the most common reason for it is ego. Pushing yourself into a more advanced pose or trying to reach as deep as more experienced yogis is going to get you hurt. You are looking for something that fits your comfort level and your skill level. It is perfectly okay to ask for modifications or to not perform a pose if you are not comfortable with it. These will all get better with practice, but suffering from injuries will hold you back, not move you forward.