Reading Json File with nested fields in go

Hi guys,

I'm very new to Go and I'm afraif the guides regarding JSON aren't very clear.

I have the following JSON output (saved as a .json file in a certain path) the content of which is

{"domain":"jsontest.campaigns.ninja","ssl":1,"server_key":"","certificate":"","bundle":"","php_ver":"5.6","wp":1,"lang":"english","start_date":"2017-02-02","end_date":"2017-03-02","description":"This is a test","user_data":{"user_id":"74","user_email":"usertest@gmail.com","user_name":"usertest"},"CampaignID":80099700}

and the following Piece of code written in Go:

package main

import (
	"io/ioutil"
	"log"
	"strings"
	"encoding/json"
	"os"
)

func main() {
	type Message struct {
		domain      string
		ssl         int
		server_key  string
		certificate string
		bundle      string
		php_ver     string
		wp          int
		lang        string
		start_date  string
		end_date    string
		Description string
	}

	var m Message
	err := json.Unmarshal(b, &m)
}

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Where b should be the path to the json file.

but as you can see, the JSON itswelf has nested fields...

My question is,
and I'm sorry if it's a long one but I really tried looking it up just can't get the concept...
How do I include the nested fields in the structure called Message and how do I read each and every value into a variable (preferably not an array but a separate variable for each value)