Once upon a time, there lived a small boy named Jack. Jack was a bit lazy, and not too bright. He lived with his widowed mother in a small house, not too far from town. They were very poor, and struggled to make ends meet.

Or...

Once upon a time, there was a widow who lived in a small house, not too far from town. She was quite poor and struggled to make ends meet. She had a son named Jack, who was a bit lazy, and not too bright.

In the first example, it seems clear thet Jack is the protagonist. But in the second example, it may be a bit fuzzy. Yes, I know it's a subtle point, but stick with me here. (Illustration by Violeta Dabija)

Generally, we meet the protagonist very early on. There may be a bit of exposition before they enter: we may see the robbers, stealing from the bank, and holding hostages until, in walks Dirty Harry, our hero/protagonist. Perhaps the one factor in example number two above is that we make the "mother" an archetypal character by not naming her. Jack has a name, so we begin to assume that he is the main character, but in some situations, it may be murky.

Most of us know that the "protagonist" is thought of as: 

The leading character or one of the major characters in a drama, movie, novel, or other fictional text.

Here's a bit of information you may not have been aware of. From the original Greek, the translation is "first actor":