Click on a username or navigate to a user's profile. (Trying to find a particular user? Read this article for instructions on how to find them.)
Click the Follow button when you see it next to a user's profile photo or on a user's profile page.
You can also click the Follow button next to any of your Who to follow suggestions, next to user search results, or anywhere else you see the Follow button on the web.
Send a text with the words Follow [username] in an SMS (text message) to Twitter to start following someone from your mobile phone. For example, text Follow jymminie to your Twitter short or long code.
You will receive a confirmation text letting you know you are now following that user.
Unsure of your short or long code? This article provides more information.