I start on Page 1, Line 1, Word 1: The main character's name.
If you know this much about the characters, the first scene introduces the main character and her status-quo world. You have 5% to 10% of the story to let your readers get to know her, how she lives her life, deals with problems, deals with other people, and what she wants out of life.
The first scene should contain a "throwaway" conflict, some minor problem or disagreement. I say "throwaway" because it may play a role in the plot, but is not itself going to change any characters. For example she wakes up on her own, which is unusual, looks at the clock and the power is out, and she has overslept an hour. Now the story is not about this power failure; the lights can come on half an hour later, but this power failure can be used to instigate something else; a chance meeting because she is not on her usual bus, or other people being late to work, etc.
I am not a plotter, I am a discovery writer with more of a loose plan. I don't begin writing until I know (in my head, not on paper) something about my main character (MC) and my secondary character (SC); the primary one she will interact with. I may have a notion of other things about my MC and SC, their love lives, professional lives, family lives, etc. But that's it, if she has lovers I don't plan them at all; I'll decide who appeals to her when I know her better through writing her.
I also know what her big problem will be (whatever drives the story), I also always have a loose written outline of how the story might end. For me this has NEVER been the actual ending. As I write, I am always checking my scenes against the ending. If the ending I had stops being viable, I stop and either come up with a new ending that will work, and then revise anything necessary to support that ending, or I revise the scene to work with what I have. Usually I revise the ending, or scrap it and come up with a better ending.
The first chapter must introduce us to the MC (often the SC can be delayed until the final scene of ACT I (20% or 25% mark in the story). Before she does or decides anything momentous, the readers need to know her, what she will be giving up, why she would feel compelled to make a life-changing decision, and so on. This is her status quo world.
This is where you do a little character building, your MC interacting with tertiary characters (that will appear a few times) and walk-ons (that will appear only once).
This is where you do a little world-building (the setting); anything in your world that would not be considered ordinary to the reader AND is important for the reader to know because it will influence the plot. Whether people take spaceships to other planets, or wear armor and carry swords, or both. Whether aliens and robots exist and everyone knows it. Whether magic works.
HOWEVER, in order for readers to remain interested, you will need conflict, which I loosely define as the reader wanting to know what happens next, in an immediate sense, of the next few pages. That is why they turn the pages, when they stop caring what happens next they stop turning pages, and will try to skip ahead or put the book down (or change the channel).
This is the entertainment factor I keep talking about in my answers, the first job of the author is to keep the reader wanting to turn pages, out of anticipation for something they know will happen soon, or for the mystery because they don't know what it is going to happen but a situation is coming to a head: Perhaps something happened and they want to know what the MC is going to do about it, how she reacts.
Now readers will give you a few pages of leeway. As a rule of thumb, I'd say 2% of the story, so in an 80,000 word novel, that is 1600 words, or about 6.4 pages (at submission format 250 words/page, which is how I write). So that is how long I have to set up; I tell myself if I don't have them wondering on page 7, they aren't going to turn to page 8.
I solve this problem by opening with my MC doing something that is physically active. Like above, having slept late and rushing through her normal wakeup routine without power, in the dark by flashlight or candle light, would be a fun problem to solve.
Because of that, the first line I write is her name followed by her doing something, so I can introduce her to the reader, and along with her, some elements of her world. Personally, I prefer opening scenes in which the MC is alone and can be herself, she doesn't have to interact with anybody. If she has a spouse I might put the MC out running, or perhaps make the spouse present but asleep for a few pages.
In any case you need to quickly get to conflict, but avoid any significant plot-point conflict with other characters, because until readers know the MC and identify with her such conflicts are going to fall flat, they won't have the impact desired.
That is the reason for the "throwaway" conflict. The reason I start there is so I, too, can get to know her by putting flesh on the bones of whatever outline of character I have devised. I have tried devising detailed characters before, no matter what I do, I find they change on the page, as they become more "real" people to me through dialogue and interaction with the world and others, so although I have a general sense of who my MC and SC are, I don't write anything, I discover them on the page.
All of that said, feel free to revise. By the end of the book, I have to do another pass to show my opening MC in their full and final personality, or the personality they should have had at that opening point. Because again, things happen on the page that may flesh out their personality more, and it needs to be shown early. For example, one of my characters became more humorous than I had originally anticipated; and I wanted to show some of her humor in her opening throwaway dilemma.