Using a new application is always confusing.
This topic has links that should be of use to you:
This topic is about a series of recently created tutorial videos for beginners:
Krita Beginners tutorial series
You can start with any tool you like. Krita has a variety of tools to create various effects and do various jobs.
I suggest that you start by selecting the Freehand Brush Tool and use it to paint in the canvas.