// The session the S3 Downloader will use
sess := session.Must(session.NewSession())

// 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 {
    return fmt.Errorf("failed to create file %q, %v", filename, err)
}

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