My answer has always been this simple: Start with a sharpie in your hand - or in your pocket, or hooked on your shirt, or attached to your belt - start with a sharpie.
(Ideally, you'll have a camera, also, and will need to learn how to ask good questions but these will be for another blog post.)
|Sharpie brand pens are The Pen - keep one with you at all times.|
The pen will help you identify developmental differences in age groups - how a three-year-old's explanation of something might sound one way compared to how a five-year-old would explain the same thing. The pen will help you begin to communicate with the class as a group, with the guardians of the children, with the wider school community.
The pen will help you start on a journey that - for me - is a privilege to continue to be on. Listening with and documenting for children is a surprise, a gift and an endless education.
Get yourself a pen. Keep it with you.
For further reading on Documenting you might enjoy:
Sophia's Train a documented art story
or "I Need Dirty Water" a 3yo makes friends using dirt and water