Altering checkout page

[Abuobaida]

Hi ,
Greeting , I'm Developer  and new in shopify platform , we have shipping systems and some of our clients deal with stores that using shopify .
What I'm trying to know if is it possible to make app and altering the checkout page specifically the price and the country and province lists by change it to ours .
we have system who calculate the delivery price according to the place was chosen in the above data . So I need after choosing the province the store send request to our external system and get the price and after completing the order send all the order data to the system.

I need like guideline or roadmap which I can follow