Do you have an injury or a physical condition that might affect your ability to practice, like pregnancy, high blood pressure, or recent surgery?

You may feel that those issues are private ones, but your teacher needs to know in order to guide you through the class safely and appropriately.
Most teachers arrive to class a few minutes early, so if you have an injury, you should, too. Seek out your teacher and ask to have a brief word with them before class. They can provide possible advice on how to approach the yoga class, and can recommend modifications for you during your practice.
Also, whether or not you are injured or just tired or stiff, skip poses you can’t or shouldn’t do, or try a modified version that avoids putting stress on the affected area. Ask your teacher if you aren’t sure how to modify a particular pose.
It’s important to let your yoga teacher know what’s going on with you, and it’s also a great way to start building a more personal relationship.