It’s up to you
It’s up to you simply means you are the one who has to make a decision.
Ravi: Should I buy this blue shirt? What do you think?
Priya: It’s up to you. You may buy whichever shirt you like.
Another piece of conversation is given below.
Vikram: I’m quitting my job.
Amar: What are you going to do then?
Vikram: I’m starting my own business. What do you think?
Amar: It’s up to you. You should know better than me.