Tonic - The first note of the scale

To remember that the tonic is the first note of the scale, recall the following:

Give yourself a tonic and you will feel better for the first time in ages.

Tonic means the first note of the scale. Whatever the scale is called, that is the first note of the scale, and therefore the tonic.

For example:

The first note of the G Major scale or tonic of G Major scale
= G