A good thing to do before writing is to know your characters so that you can write from their perspective and what they would do, not you. I know that sometimes this is difficult; I struggle with this a lot.
I like to create a new part in the story and I do not publish it. Repeat: do not publish it.
After creating a new part, I make a section titled "BASIC." Then, I like to list the important characters, and the basic information about them.
My next section is titled "BACKGROUND." Again, I list the important characters. This time, I write about their background stories.
The third section is called "PERSONALITY." This is where I put the personality about the characters and what they like to do.
I also like to do a "FRIENDS" section and an "ENEMY" section. These are both self-explanatory. If they have boyfriends/girlfriends, I usually put them in the "FRIENDS" category and specify that they are a boyfriend/girlfriend.
I also like to do an "OTHER" category for information that doesn't fit with the rest.
EXAMPLE:
BASIC
Brynn
Thirteen years old.
Girl.
Nicknames: BKendall
Fourteen years old.
Girl.
Nicknames: KK, Kendall K, KBACKGROUND
Brynn - overprotective parents, always been a goody-two shoes, wanted to be rebellious but can't because of parents, pretty good childhood
Kendall - dad died when she was 6, mom almost never sober, had to take care of herself for most of her life, picked on at school
PERSONALITY
Brynn - usually pretty happy, wants to be a rebel but can't because of parents & it doesn't suit her personality, not a "popular," but not picked on, does good in school, like so play basketball, creative
Kendall - always seems depressed but doesn't have actual depression, fails in english and history, doesn't defend herself much, always seems to find the negative things in life & things
ENEMIES
Brynn - Maddie because Maddie is in volleyball and thinks basketball players are stupid, Paige because Paige started hating Brynn for an unknown reason
Kendall - Chloe because Chloe bullies Kendall because of her Kendall's dad, anyone who is friends with Chloe
FRIENDS
Brynn - Kalani
Boyfriend? No.Kendall - none currently
Boyfriend? No.And that's how I do it! Sometimes it varies depending on the story. I don't really have a fancy acronym like @zeeglur does, but if you can make one uo, that would be awesome!
I am not saying that you have to do this, but it is a good thing to do. You also are not obligated to do this exactly like I do; be creative and make it easy for you to do!