How do I make my characters stronger?

just get stronger characters.

but you have to know what make people mentally strong or weak and physically strong or weak

when you become a semi expert in that, you will know how to portray your characters.

find a few people and read their biographies. you will be inspired.

http://americanfolklore.net/folklore/2010/07/john_...

http://dig.lib.niu.edu/gildedage/pullman/events3.h...

there are plenty of people with strong characters, leadership, dared to take responsibility, dared to sacrifice themselves... and expected nothing for themselves.

You must put obstacles in the way of the character (male or female) and then with struggle let them overcome problems. One thing young girls do is write about relationships and feelings too much, especially if you have characters just talking about how they feel. Not many people want to read 300 pages of feelings. You need, perhaps, more action in your stories. Sex should also be part of the story, even for a young girl. Not sex scenes. I don't know how old you are but I know you think about sex and love. Use some of that. .