“It's up to you.”
Your mother is going to make breakfast. She asks you what you want to eat. You don't care, so you say this.
It's up to you.
You say "It's up to you" when someone asks you to make a decision but you want them to decide:
A: Which one should we get?
B: It's up to you.
When a decision is "up to" someone, that means that they are the one who can decide. You can talk about who has the power to make a decision:
I'd like to help you, but it's not up to me. You'll have to ask Karen.
"It's up to you" is a pretty polite way to let someone else make a decision. A more direct and less polite way is:
I don't care.
And a more formal and polite way is:
Whatever you prefer.