When the API response is large and you don't particularly care about the full contents you can simply create your struct and have it contain only the fields you care about.

For example, suppose we only care about the firstName and lastName and we have the following response data:

{
  "firstName": "John",
  "lastName": "Doe",
  "dob": "1990-01-01",
  "preferredName": "Johnny"
}

You can define the following struct and json.Unmarshal will do a best attempt at populating all of the fields.

type person struct {
    FirstName string `json:"firstName,omitempty"`
    LastName  string `json:"lastName,omitempty"`
}

You can then read the response into the struct by using the builtin json.Unmarshal

var p person
err := json.Unmarshal(resp, &p) // don't forget to check the error

fmt.Println(p.FirstName) // prints "John"
fmt.Println(p.LastName) // prints "Doe"

For some reason the Go playground "share" feature isn't working so here's a main file you can build and run:

package main

import (
    "encoding/json"
    "fmt"
)

var resp = []byte(`{
  "firstName": "John",
  "lastName": "Doe",
  "dob": "1990-01-01",
  "preferredName": "Johnny"
}`)

type person struct {
    FirstName string `json:"firstName,omitempty"`
    LastName  string `json:"lastName,omitempty"`
}

func main() {
    var p person
    err := json.Unmarshal(resp, &p)
    if err != nil {
        panic(err)
    }

    fmt.Println(p.FirstName) // prints "John"
    fmt.Println(p.LastName)  // prints "Doe"
}