Developing a solution for blocking a checkout process by postalcodes / zipcodes

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Hello everyone,

Just started reading the documentation on shopify developers and quickly got overwhelmed by so much information and options that are available.


My goal is to create a step on the checkout process where only delivery addresses with a specific postal code / zipcode can be completed. 
I already looked into available apps, but a lot of them lock the "add to cart" option but the user can always change the address on checkout so it's no good. Also being a developer, I would like to learn more and implement this myself.

Now the question is:
- Where to start? What is the API or solution that shopify give us so I can try and create something to achieve this goal?
I don't want to create a whole storefront and checkout just for this since i woud like to continue to use the themes aspect of shopify.

Could any1 point me in the right direction? 

Thank you very much!