My main piece of advice that isn't just repeating the excellent advice already given about planning everything out is this:
Be at peace with and be ready to use the fact that the vast majority of your readers won't care about the plot itself, but how that plot affects the characters. A clever plot is not in itself compelling to most people just by virtue of being clever, wheras characters and emotional stakes are.
I see people pretty regularly making these very complex plots where some mastermind characters scheme over the fate of a kingdom and wondering why nobody's reading, and they all too often blame readers for being stupid, rather than looking in the mirror and asking:
- Does the fate of the kingdom affect my protagonist's life in a clear, concrete way?
- Are all these things that are happening having obvious consequences for my characters?
Because if not, it's a bunch of people talking about abstract ideas that are sort of... interesting hypotheticals rather than a story about a journey or trial that happened to a person with some kind of a point and a conclusion.
If Chancellor Xordak has schemed to have the royal family or the kingdom of Grandus assassinated through corrupting the five dragon spirits that dwell within the temple of Blor by exploiting their ideology against them, that's clever, but it's only compelling if say... the main character of this story is one of the royal family of Grandus and they're next on the list to be assassinated having lost their parents and siblings and now they want to put a stop to Xordak's schemes, or say, the protagonist is a young acolyte of the temple of Blor who was friends with one of the dragon spirits that was corrupted and Xordak doesn't realise his plan to make himself ruler of Grandus will also cause the corrupted dragons to attack all the cities on the continent, threatening our hero's friends and family.
If you think about Game of Thrones or Code Geass or Death Note or Fate/Stay Night or Hamlet, they all have complex plots, but they're complex with stakes that threaten the protagonists and events that cause things to happen that change their status and make them react to new challenges.
If you think of say... The Spider-man clone saga on the other hand, it's incredibly complicated, but a lot of what's happening seems disconnected from the main characters, and it's hard to know whose story it is because it keeps flip-flopping on who is the real spider-man (so the readers don't know who they should be emotionally invested in), hence people got really burned out on that storyline.
Or hey, here's another example: Star Wars Episode I has a way more complex plot compared to Star Wars Episode IV, involving a politically motivated blockade of a planet to disrupt trade due to the machinations of a shadowy figure who wants to manipulate the democratic senate in a way that will slowly erode it into a dictatorship under him, meanwhile two knights are doing an investigation that gets sidetracked and they end up trying to broker an alliance between the planet's dominant culture and the marginalised people who live there, freeing a slave and trying to train him as a knight, uncovering a conspiracy... oh my god... compared to "A farm boy meets an old wizard, rescues a princess and stops the enemy superweapon from blowing up his new friends". I is way more complex, but the better movie is pretty much universally agreed to be IV, the one where you have a clear sense of who the protagonist is, what the stakes are and who will be directly affected by stuff that happens.
Always keep in mind the question of "How is this going to make my protagonist's life difficult?" not "How is this going to show readers how smart I am?"