config.json

{
    "db_url": "127.0.0.1",
    "db_port": 8080,
    "db_username": "foo",
    "db_password": "bar"
}

main.go

package main

import (
	"bufio"
	"encoding/json"
	"os"
)

type dbConfig struct {
	Url      string `json:"db_url"`
	Port     int    `json:"db_port"`
	Username string `json:"db_username"`
	Password string `json:"db_password"`
}

func main() {

	f, err := os.Open("./config.json")

	if err != nil {
		panic(err)
	}

	r := bufio.NewReader(f)

	decoder := json.NewDecoder(r)

	c := dbConfig{}

	err = decoder.Decode(&c)

	println(c.Url)
	println(c.Port)
	println(c.Username)
	println(c.Password)

}