Well, here is one way to look at it.
You have a start and an end state of your book.
He is the third son of an impoverished baron
He will be the hero who slays the demon king
Break that up into some larger points of how your MC gets there, not necessarily in this order or only a single time
1 He needs a reason to leave his home
2 He needs training
3 He needs a weapon
4 He needs a party
5 He needs to protect people to show he is indeed a hero
Then break these up and form the basis of your story
The town is ransacked by the demon king's army, driving him out in nothing but his nightclothes and a small knife
He vows vengeance for his family and his people
He goes to petition the king to do something about it
gets turned away
Gets captured by bandits and is to be sold into slavery
Is rescued by an old man who beats the ever living tar out of the bandits,
Asks to train under the man
long training montage, meets another disciple who will end up being a party member
They get attacked by demons again, and the old man sacrifices himself so they can escape.
They noticed they couldn't hurt one of the demons with their current weapons
That is point one and point two accomplished, and gaining point four. Also leads to the next step of point three
Heads to a larger town to find a blacksmith, whom they end up rescuing from some gangs.
Blacksmith says they need to find X mineral for him to be able to forge X weapon that is effective against demons
Sends his child/apprentice with the MC to help locate said Mineral
accosted by monsters, unable to get mineral X because they are too strong
Finds a mercenary/adventurer's guild to hire help, and ends up doing a few jobs themselves to acquire the help
End up rescuing a powerful adventurer who had been captured. They agree to help fight the monsters
Gain the minerals
Weapon created
This will give you points in four and five, allowing you to acquire your three
Hears about the nearby town about to come under assault from the demons; he rushes there.
Saves the town, hears about captives, and mounts a rescue operation
follows the demon army back towards their territory, sneaks into the camp
Rescues captives, but is caught while covering their escape.
Held in captivity for some time, trying to learn what you could from your jailers, over hearing Critical information at some point
Rescued by the adventurers from that town, you helped
This will give you a lot more credibility for point five and for the information you gain to reach your end state.
Reunited with your party, heroes welcome for saving those people.
King wants to meet with you
awarded demon slaying weapon X
You and your party march with the kingdom against the demon army
You slip behind their lines, intent on assassinating their leader
From here, you can finish your story, or set yourself up for a failed attempt, or possibly convert some enemies to your side instead and continue your story in a slightly different direction.
This isn't just for fantasy; you can adapt it for any writing, really. Just keep breaking it down into a storyboard to map your plot points. Don't go too deep, or else you might write yourself into a hole.