Characters are one of the biggest part of a story. You need to figure everything out for them! Their personality, their looks, who they marry or if they don't get married, who their friends are or maybe they don't have any friends! Characters help form the story.
Here's a chart that I found that ask you question about your character. It'll help because it has what they reader should know about them:
I suggest you write these down somewhere so you will remember. And when you are planning a character look back at it and see if you got most of them down.
Tips to remember:
1.Don't make a chapter about your character telling everything about them. Let the reader find out for themselves as the story moves on.
2. Makes all your characters with different personalities so your readers are draw to one and feels like they are that person. You don't have to make a lot but you can make a character an emo person and then another can be girly or something.
3. If you need help with names you should look up girl names and start making a list so if you need to come up with a name for a character then you can look at the list.
Hope this helps! Comment anything you have trouble on and I will try to help.