下記を参考にした
https://docs.aws.amazon.com/sdk-for-go/api/service/s3/
https://github.com/awsdocs/aws-doc-sdk-examples/tree/master/go/example_code/s3

package main

import (
	"fmt"
	"os"

	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/credentials"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/s3"
	"github.com/aws/aws-sdk-go/service/s3/s3manager"
)

func main() {
	creds := credentials.NewStaticCredentials("アクセスキー", "シークレットキー", "")

	sess := session.Must(session.NewSession(&aws.Config{
		Credentials: creds,
		Region:      aws.String("ap-northeast-1"),
	}))

	filename := "aaa.txt"

	// Create a downloader with the session and default options
	downloader := s3manager.NewDownloader(sess)

	// Create a file to write the S3 Object contents to.
	f, err := os.Create(filename)
	if err != nil {
		fmt.Println(fmt.Errorf("failed to create file %q, %v", filename, err))
		return
	}

	// Write the contents of S3 Object to the file
	n, err := downloader.Download(f, &s3.GetObjectInput{
		Bucket: aws.String("backet-name"),
		Key:    aws.String("/aaa.txt"),
	})
	if err != nil {
		fmt.Println(fmt.Errorf("failed to download file, %v", err))
		return
	}
	fmt.Printf("file downloaded, %d bytes\n", n)

}

// ついでにアップロードは下記
func upload() {

	accessKey := "your accessKey"
	privateKey := "your privateKey"
	region := "ap-northeast-1"
	fileName := "gazou.png"
	bucketName := "your bucketName"

	creds := credentials.NewStaticCredentials(accessKey, privateKey, "")
	sess := session.Must(session.NewSession(&aws.Config{
		Credentials: creds,
		Region:      aws.String(region),
	}))
	uploader := s3manager.NewUploader(sess)

	f, err := os.Open(fileName)
	if err != nil {
		log.Fatal(fmt.Errorf("failed to open file %q, %v", fileName, err))
	}
	res, err := uploader.Upload(&s3manager.UploadInput{
		Bucket: aws.String(bucketName),
		Key:    aws.String("/test/" + fileName),
		Body:   f,
	})
	if err != nil {
		fmt.Println(res)
		if err, ok := err.(awserr.Error); ok && err.Code() == request.CanceledErrorCode {
			fmt.Fprintf(os.Stderr, "upload canceled due to timeout, %v\n", err)
		} else {
			fmt.Fprintf(os.Stderr, "failed to upload object, %v\n", err)
		}
		os.Exit(1)
	}
	fmt.Println("successfully uploaded file!!")
}

exampleに、credentialsの部分の記述が見当たらなかったので、メモ的にアップしました。

それ以外はサンプル通りです。結構簡単ですね。