A paragraph is a group of sentences about one idea. It is the smallest piece of real writing English asks you for — smaller than a letter, smaller than an essay, and still a complete thought.
A group of sentences
My mornings are quiet. I drink tea by the window. The street is still empty. I like that hour before the day starts.
Four sentences. One picture. That is a paragraph.
One job
If you change the idea in the middle — tea, then your job, then a film you saw — you have started a new paragraph without meaning to. Keep the job small, and finish it before you begin another.