My answer to The Psychology of Starting a Piece of Writing is a guide to getting started. Other answers there are good as well. This will explain how to get going on the first line, first scene, first chapter.
Organizing Your Ideas.
I suggest creating four piles, for what I consider the four parts of the story (each about 25% of the book).
Part 1: Ideas about introducing the characters, the setting, and the "Normal World", before the main problem shows up. We need to know them before the main action begins, or we don't care about them. So we introduce them, and probably who or what they love, why they will take risks and suffer hardships. Halfway through this part, you introduce the "inciting incident", and any ideas about dealing with that begin here. Obviously, the inciting incident is the first signs of the main problem driving the story, and in many cases the MC doesn't recognize it as a BIG problem, and fails to address it (or makes matters worse) by trying to deal with the inciting incident as a little problem.
Part 2: New Complications, ideas about how the characters encounter or create new difficulties. The story gets more complex here, perhaps due to failures in their initial attempts to deal with the inciting incident. Or they find out it is worse; the cancer has spread, the corruption is deeper than they thought, the mentor they trusted is a mole, their name and signature has been forged on all the damning contracts, the person they just had a shouting match with, in a public restaurant, has been killed.
Part 3: Unwinding Complications. Your ideas about resolving the problems. Small problems get resolved, then larger ones, and we start finding ways to uncomplicate the story. By reconciliation, negotiation, or violence. The heroes can still make mistakes, but the complications and their resolutions are teaching them about dealing with the problems. By analogy, imagine you are plucked from your normal life and dropped in a jungle. Part 2 is about you frantically trying to survive. People around you die, attacked by wild animals, drinking bad water, eating bad food. But if you don't get killed, these incidents have taught you something, and you move into a new phase (Part 3) of understanding this dangerous new world, and navigating it more safely and expertly. The jungle is becoming more predictable.
Part 4: The "big problem" still exists, but here we have learned enough, we finally discover the key to the resolution. But it will require a big risk, and we have one chance at this. We take it, for the sake of what we loved in Part 1. In my books this is a success, and then the heroes (those still alive) return to their New Normal world, changed for the better. More mature, or in love, or less selfish, more responsible, etc.
Take your ideas and put them in order, how they might contribute to the story. sections that are light need work, and more ideas. To me, the most important section is the beginning, I spend a lot of time thinking about my characters and trying to make them real people in my mind. I also think about the big problem, and how it might be resolved, so I have a "destination" for the story in mind, but it isn't ironclad, as long as I always have some possible ending in mind.
I think if you imagine real people and put them together, and start throwing problems at them, they will react and you will get a story out of it. If you throw something that blows up the story, sends them off on a tangent you don't like, then just undo it, and try something else.
Get your ideas organized by what part of the story they would probably appear in. Keep those ideas as notes, things you could fit into the story, or want to find a place for in the story. You may not use all of them, characters moving through their life may give you new ideas, you should keep them too, as potential scenes.
But that said, I write the book from the beginning and go to the end, I don't skip around. Every chapter depends on the characters up to that point, and they are changing (at least in what they know, perhaps in personal ways as well), so (for me) I can't really write Chapter 10 until I have written Chapters 1-9. My characters are not robots.