One of the primary functions of the first act is to establish the current situation of your protagonist. You, the writer, have to convey to us, the reader/audience, what’s going on around our hero and how they fit into this scenario.
Tell us what we need to know about them, but don’t overwhelm us. Give us the details relevant to the story; anything else is totally unnecessary.
Show us why this character is worth our time and attention. What is it about them we can relate to? Make us care about them. Pique our interest and curiosity.
This is also your chance to establish the rules of this world you’ve created and how it works. If it’s something other than the world as we know it (i.e. science fiction or fantasy), then you definitely need to explain how things work here. Don’t expect us to know what you’re talking about; just because you know doesn’t mean we’re going to. Keep things simple.
Now that you’ve presented us with our protagonist and the world they live in, SOMETHING HAPPENS around page 10 that drastically changes their everyday routine and gets the story going. It also raises the central question of the story – “Will the protagonist achieve their goal?”
Some may say the protagonist should be the one that makes something happen, which is possible, but it seems more likely this disrupting-of-the-everyday-routine event happens to them, rather than because of them.
What’s great about this next stretch of pages is it offers you the opportunity to show a little more of this world, especially how your protagonist reacts to what’s happened to them as well as how their world responds to it. Explore the consequences and ramifications of how their life is changing as they leave their old situation behind and become more involved with the new one.
By the time we get to the end of the first act, the protagonist is totally immersed in this new world and has to figure their way out/through it. Everything we need to know has been taken care of, so now we can focus on following the story.