Here is a simplified version of part of 'Minuet in G' written by J.S. Bach.
A very well known piece, popular with beginners of all ages.
In this lesson, instead of playing chords with the left hand, the piece uses single notes which harmonise with the right hand melody.
The numbers written alongside some of the notes, are 'fingering numbers', that is, they tell us which fingers to use, to play those notes, more easily and comfortably.
You may notice that in the 3rd bar of the piece, that the green E note, is played with finger 3. This makes it easy to play the notes which follow, ending up with finger 5, playing the high G.
Also, in the 7th bar, we need to cross finger 2, over finger 1, again to make the playing easier.
Practice playing each hand separately, before trying to play with both hands, while counting out the beats, and watch out for the F Sharps
Remember, from the key signature, that this piece is in the key of G major, which is why it is known as 'Minuet in G' :-)
Study the video carefully, to see the fingering as I described above.