How to unmarshal objects inside array

I am trying to get access to object's values inside of array

[
  {
    "name": "London",
    "lat": 51.5073219,
    "lon": -0.1276474,
    "country": "GB",
    "state": "England"
  }
]

I use this code to unmarshal it

content, err := ioutil.ReadAll(res.Body)
	if err != nil {
		log.Fatal(err)
	}

	var data []ResponseData
	err = json.Unmarshal(content, &data)
	if err != nil {
		log.Fatal(err)
	}

This is my struct

type ResponseData struct {
	Name       string      `json:"name"`
	Lat        float32     `json:"lat"`
	Lon        float32     `json:"lon"`
	Country    string      `json:"country"`
	State      string      `json:"state"`
}

I need to simply fmt.Println(data.Lat, data.Lon) later.

The code you presented should unmarshal your JSON successfully; the issue is with the way you are trying to use the result. You say you want to use fmt.Println(data.Lat, data.Lon) but this will not work because data is a slice ([]ResponseData) not a ResponseData. You could use fmt.Println(data[0].Lat, data[0].Lon) (after checking the number of elements!) or iterate through the elements.

The below might help you experiment (playground - this contains a little more content than below):

package main

import (
	"encoding/json"
	"fmt"
	"log"
)

const rawJSON = `[
  {
    "name": "London",
    "lat": 51.5073219,
    "lon": -0.1276474,
    "country": "GB",
    "state": "England"
  }
]`

type ResponseData struct {
	Name    string  `json:"name"`
	Lat     float32 `json:"lat"`
	Lon     float32 `json:"lon"`
	Country string  `json:"country"`
	State   string  `json:"state"`
}

func main() {
	var data []ResponseData
	err := json.Unmarshal([]byte(rawJSON), &data)
	if err != nil {
		log.Fatal(err)
	}

	if len(data) == 1 { // Would also work for 2+ but then you are throwing data away...
		fmt.Println("test1", data[0].Lat, data[0].Lon)
	}

	for _, e := range data {
		fmt.Println("test2", e.Lat, e.Lon)
	}
}