Steam is a video game digital distribution service developed by Valve. It was launched in September 2003 as a way for Valve to provide automatic updates for their games, and has since expanded to include a wide variety of games from other developers and publishers.
To go online on Steam, you will need to create a Steam account and install the Steam client on your computer. Once you have done this, you can launch the Steam client and sign in to your account. Once you are signed in, you will be able to browse the Steam store, purchase games, and play games online with other players.