You can left-click to place a portal anywhere you want. Within reason.
Within reason?
Generally, portals can only be placed on white surfaces that have plenty of space for them. Otherwise, it's fair game - you can put portals on walls, ceilings, and floors alike!
So how do I get out of here?
The door blocking the exit opened when you grabbed the portal gun.
There's a portal next to it, too.
Put a portal anywhere you can reach, then walk through it. You can then enter the elevator to reach the next chamber.
Guide help
This is the low-spoiler guide for "Leaving the chamber" puzzle in Portal. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.
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