Developing characters can be daunting. You have to figure out the most important details that will be essential to the character's development throughout your story. Here's my ultimate basic questioner on how to develop your characters.
1. First, middle, and last name
2. reason for these names
3. nicknames
4. appearance (hair, skin, special features, etc)
5. birthday
6. age
7. role in the story
8. personality
9. favorites (songs, books, movies, etc.)
10. mindset during the beginning of the story
11. mindset at the end of the story
12. what causes these changes
13. family
14. people they're closest to
15. relationships (family, friends, romantic, etc.)
16. character arc
17. aesthetic
18. theme song
19. likes
20. dislikes
Whew! That was a long list, but it is the ultimate basics on how to get to know and develop your characters. I, personally, use this list as well. To go even further, I would recommend making Pinterest boards, taking personality quizzes as your characters, and listing similar characters. Of course, adding some extra question is 100% recommended. Take your time to know your characters. It will help greatly.