As a beginner learning how to do ballet, you should know to take things slow. You won't become the best overnight and you shouldn't try. When you start off doing ballet you will not be as strong as you should be. This is normal. In the beginning you will be learning the basics which prepare your body for these new moves. You have to gain strength in areas that you never had to before. So give yourself plenty of time to learn things and to prepare your body for this dance.
Stretching is the most important thing when you start ballet dancing. It's especially important for beginners because you don't know what your body needs right now. That doesn't mean a pro shouldn't stretch, because if they didn't, they wouldn't be a pro right now. Without stretching you will pull something, fracture something or even break something. You should begin each session with a good stretch. It should include stretching every part of your body.