https://www.musical-u.com/learn/finding-tonic-major-keys/
Do you know how to find the tonic?
From passionate amateurs to professional musicians, finding the tonic is the key to playing or singing in tune. Whenever you cannot understand what’s going on in a song (a secondary voice you can’t hear, a harmonic progression you don’t understand) going back to the tonic helps you find the right pitch and understand the context of whatever element you are trying to grasp.
The first step is to educate your ears. The ears are part of your musical instrument and you can help them with your voice.
Musical-U tells you exactly how to find the tonic and the many benefits of this skill.