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.