Summarize this article:

Because this only works if the Tweet is much shorter than 280 characters, it's recommended that you use the official retweet button instead. Still, this is an effective method of quoting and reposting a tweet. After you've copied the content, open a new Tweet box and paste the content into the message. This shows that the content is a retweet and isn't your own content. This is the most courteous way to borrow the content and tip your hat to the original author. You can also add your own comment at the beginning of the message. Example: Wow! RT (@username) "Quoting a tweet is so easy."

Summary:
Copy the content of the Tweet you'd like to share and retweet it. Add the letters RT and the Tweet author's @username. Click Tweet to post the message to your followers.