Here's a new idea: ask your protagonist what she has to hide. Does she use drugs? Is she a kleptomaniac? What is the one thing she doesn't want people to know about her? The secret doesn't need to become the center of the story, although if you're looking for a central struggle, a protagonist living with big secrets like porn addiction or a secret agent past make for great central tension. The secret could be as small as, "I never told my mom I actually hate her sushi. She makes it for every special occasion just to please me, and I'm too afraid of hurting her feelings to say anything about it." A little secret like that can add comedy to a scene, or a little more drama to a major incident. If this girl finds herself yelling at her mom and reveals her hatred of her mom's cooking, you'll find sometimes that such lines add genuine "straw that broke the camel’s back" stresses to a relationship.
The internal conflict of keeping the secret also juxtaposes nicely against the external tension with other characters. If your leading lady has a deep-seated addiction, she will fight other characters to hide it. She may even physically stop someone from going into her room, or out of defensiveness punch someone who accuses her. She will also fight herself constantly. Maybe she wants to get close to character B, but she fears he won't respect her if he knows about her secret. She has to struggle between her desire to trust B, and her fear of his disapproval. She may even keep the secret from herself in some ways: perhaps she's gone into denial about her problem and refuses to admit it exists.
In the end, the secrets your character keeps can change her image or her world if revealed, so the way your character deals with secrets shows something about him or her. Even little secrets like past crushes or a guilty desire for cultural-taboo foods can foreshadow passivity or fear in a character, giving you another "show, don't tell" opportunity with your readers.