Before starting a story, I have a rough skeleton of the plot. That means that before even starting to write the chapter, I usually establish several things:

• the main cast and the important supporting characters
• your main character's defining traits and power set. For example: [in my next fic, Kyuubi secretly shares his Negative Sensing Emotions with Naruto when he is very young. Because of that, he gets to feel the villagers' dislike and hatred for him much more vividly which in turns change him from the happy-go-lucky person he used to be. He will specialize in Earth Release.]
• (preferably) the pairing. At the very least, it's better if you make up your mind before you reach the length of 10-15k words about whether your fic will have romance or not and if it does have, with what characters. It's better to give yourself time to plan their encounter and to develop their relationship gradually.
• the main story arcs. This is very important. They don't need to be super detailed but I need to have a rough direction and several notes. Here is a super generic example:

1. first arc - graduating from academy (this is the exposition)
2. second arc - wave mission (notes: sasuke dies, sakura gives up on being a kunoichi. Naruto is the only genin left from Team 7 which disbands)
3. third arc - Naruto gets a new team and go on one (or several) missions. (if you were planning on pairing him with an OC this is the perfect moment to introduce them)
4. fourth arc - Chunnin exams (note: takes place in Kumo. Someone attempts to assassinate your main character) ETC ETC.

At least this is how I plan my stories. As for details such as the action scenes, the interactions between characters and the number of characters I'm going to kill off, I generally leave that in the air because I often get amazing ideas on the spot, while writing a chapter.