Who's the hero of your story? Who's your protagonist?
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.
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":
Back in the day (no, I wasn't there, but sometimes I do feel that old!) the audience sat high on the hill at the theatre. It was difficult to actually see the actors' faces. Most were dressed in plain, non-descript costumes. Often, they had large masks that evoked a specific emotion. But in most cases, the protagonist was the "first actor" to come on stage. They were the hero (or heroin) of the play. They were the protagonist.
I mention this only to help you understand where your focus is in your story. There are times when more exposition at the beginning is necessary to set the scene, or convey a specific feeling. But remember who your hero is, who is the main character? Be careful about introducing lesser characters first. You want the protagonist "up front" in the minds of your audience.
There are exceptions to every rule: It may be useful to introduce the antagonist first, to help us understand what the main character has to deal with. Describing the dragon in great detail may help us understand what our hero has to overcome.
This tip, like all others, is not the end-all and be-all. It's to get you THINKIING about how you craft your story!