Formula...

Hello mommas!

I am currently breastfeeding my daughter and, I am wondering when can I start her on formula. I only have 3 weeks left before I go back to work. She will be 3 weeks tomorrow and I have not given her any formula since she was born. I am planning on doing both, formula when I am not with her and breast milk when we're together. Do any of you know which formula is the best so her tummy won't be irritated or get colics?

Thank you for reading, I would really appreciate any advice.