Welcome to the Krita Manual! In this section, we'll try to get you up to speed.
If you are familiar with digital painting, we recommend checking out the Introduction Coming From Other Software category, which contains guides that will help you get familiar with Krita by comparing its functions to other software.
If you are new to digital art, just start with Installation, which deals with installing Krita, and continue on to Starting Krita, which helps with making a new document and saving it, Basic Concepts, in which we'll try to quickly cover the big categories of Krita's functionality, and finally, Navigation, which helps you find basic usage help, such as panning, zooming and rotating.
When you have mastered those, you can look into the dedicated introduction pages for functionality in the ユーザーマニュアル, read through the overarching concepts behind (digital) painting in the 一般的なコンセプト section, or just search the リファレンスマニュアル for what a specific button does.