To locate your phone online, use a mobile tracking app accessible from a web browser such as Find My iPhone and sign in to the application. The exact method of tracking your phone online depends on the phone device used and mobile or Web application used.
If you own an iPhone, use the free Find My iPhone app by signing in at iCloud.com. This app allows you to send a message to your lost cellphone that can inform someone who finds your phone how to contact you. You can also remotely erase stored information off of your mobile device. The Tile app is accessible from a web browser and can show you the last location of your phone when you sign in.
You can also use the Lookout app to track the location of your phone remotely online if you use an Android device. This application pinpoints where the phone is on a display map.