Even if you don't feel you are very flexible, yoga can still be for you. Flexibility in yoga is rooted in the understanding of our physical limitations limited only by our mental boundaries. Your mind may be the one thing holding you back from stretching a little bit deeper and lengthening just a little bit longer. Peak performance is only a few sessions away. Once you release the mental element and negativity from your mind, you will find obtaining "pretzel-like" positions are much easier than you thought.
Probably the hardest thing to convince you of with regard to yoga is that there is no wrong way. I can't tell you how many students feel like they don't belong in a yoga class because they aren't as flexible and can't keep up with the rest of class. There is no wrong way to do yoga. It doesn't matter how low you can go, how deeply into a stretch you can hold or how fast you can get into one position or the other. We have physical weaknesses but by the same token we have physical strengths! This is the good news about yoga. You are about to learn much about yourself!
You will begin to learn which poses in yoga really "speak" to your body. By this I mean you may feel yourself getting more and more flexible in certain poses. From there, you can improve a.) your form and b.) the intensity and duration you're able to hold that pose. Because our bodies are different, stretches for you may come easier than others around you. Recognize that there are going to be poses you are better or worse at than your neighbor, there are poses that will empower you. Make a mind-body connection between what feels like it's still progressing versus what you feel you have mastered.
As a beginner, it is especially important to note that modifying positions is not only perfectly acceptable but should be encouraged by your instructor. Never force a stretch or pose. Yoga is a progressive and learning process, not to be rushed. Keep joints strong, but not locked, knees bent instead of straight and shoulders away from the ear lobes no matter how close to mat you are able to fold over.
"Form before Floor!" and the rest shall simply fall into place.