1
Open your favorite Web browser and log in to your Twitter account with your username and password.
2
Scroll down your Twitter homepage to view all of the latest updates from people you've been following until you find the post that you want to comment on.
3
Hover the mouse over the post so that the various options, including "Reply" and "Retweet," appear in the top right of that post.
4
Click "Reply." A window will appear including a text box with the "@" symbol followed by the user's name in it.
5
Type in your comment after the existing text. Ensure that you leave a space between the beginning of your comment and the user's name. Note that "Tweets" are limited to 140 characters -- this includes the @ symbol and the user's name.