Quick google:
Instructions
1. If you already know which Tweet you want to link to, skip to Step 3. If you need to make a Tweet, read on. First, go to
http://twitter.com/ and up at the top right corner, click on Sign In. Enter your user name and password.
2. Now it will take you to your main page. Up at the top where is says What Are You Doing? this is where you enter your Tweet. Type in whatever you want to Tweet, up to 140 characters, then hit Update. Your Tweet will appear below with all your Followed Friend's Tweets.
3. Look at the Tweet you want to link to. You will see a time below it, such as 'about 2 hours ago' or 'less than 5 seconds ago', etc.. Just click on that time and it will take you to your individual Tweet. Now just copy the link out of the address bar and you have your Tweet link!!