Any idea what plant is this?
I have no idea how to start


You probably need an "identification key", or some kind of guide, that will help you identify the plant based on a series of questions or identification criteria.

To be able to use such keys or guides, you will need to familiarize yourself with technical vocabulary used in plant description. Some idea of the kind of ecosystem where the plant lives in would likely help too.

If the plant seems common, you will maybe find it on listings of common plants for your area.