You cannot start Steam without logging in. You need to either log in with a username and password or with saved credentials. You cannot login to Steam with a username and password without an Internet connection. You cannot save credentials without first logging into Steam with a user name and password. You cannot start Steam in offline mode without saved credentials.
What this all means is that in order to start Steam in offline mode, you need to have previously connected your PC to the Internet and successfully logged in to Steam. The "Don't save account credentials on this computer" setting must have been turned off when you did so. If you haven't already done this then there's nothing you can do to start Steam without an Internet connection.
If you are ever able to connect your PC to the Internet then you should follow Valve's instructions on how to use offline mode:
Using Offline Mode on a PC.
Please follow the instructions below to configure Offline Mode on your PC:
- Start Steam online - make sure the Remember my password box on the login window is checked
- Verify that all game files are completely updated - you can see the update status for a game under the Library section (when the game shows as 100% - Ready it is ready to be played in Offline Mode)
- Launch the game you would like to play offline to verify that there are no further updates to download - shut down the game and return to Steam once you have confirmed that the game can be played
- Go to Steam > Settings to ensure the Don't save account credentials on this computer option is **not* selected
- From the main Steam window, go to the Steam menu and select Go Offline
- Click Restart in Offline Mode to restart Steam in Offline Mode
All the other answers you may have seen are solutions for if you didn't perform steps 5 and 6. There's nothing you can do to avoid having to perform the other steps, as they're part of Steam's method of copy protection. Requiring that you log in at least once and validate that your account actually has purchased the games installed on your PC is how it tries to limit piracy.