Student or Learner
If you're happy and you know it, Clap your hands. After a comma, we do not start with a capitalization. Why is it capitalized here, please?
It is part of a song lyric, and these are conventionally treated as verse, the poor cousin of poetry.
It should be written as in your attachment:
If you're happy and you know it
Clap your hands.
In verse and poetry, the convention is that each line starts with a capital letter.