All my supporting characters are really cool and have their habits and problems and backstories but I just feel so lost when it comes to my main character. She just seems to be floating around in the story, not doing anything that sets her apart. She is a volleyball player who wants to get better but is on a lazy, not serious team. I just don't know what to do with her?
I've always found it useful to answer a few questions about the character like I'm interviewing them. Imagine what they would say and how they would respond. Maybe you'll learn a little bit about them.
What are your MC's passions? You said she is a volleyball player, what motivates her to play? Does she find it fun or is there some deep story behind that? Did her father/mother play? Is she trying to be better than her older sister?
What other hobbies does your character have?
What are some flaws? Is she too talkative, bubbly, quiet, shy/awkward, confident, etc.
You said your MC doesn't do anything to set her apart. MAKE her stand out. Make her interesting.
However, the MC will automatically connect more with the reader since we're seeing the story through their eyes, so even if your MC isn't super interesting or unique, it's not a huge problem.