Im trying to use the skybox that is in use when escaping from wheatley at the part where he kills you. Ive never used a skybox before and i cant seem to do it. Help!

Pretty much, rules:

Do not just surround your map with a giant skybox and then delete parts of your walls to make sure it's sealed, this will completely screw over vbsp and vvis by creating extra visleaves.

Make sure you map is already sealed, and then texture inside faces you want the player to be able to see the sky at with the tools/toolsskybox texture. Its little texture icon should say "Skybox" on it. Next, in Map Properties, in the "sky" parameter, enter the name of the sky texture. I'd recommend decompiling that level to see what skybox they used.

Actually, it's not a skybox. You literally just wall it is and texture it with a black texture. With the correct fog settings, it looks just like the campaign. However, I'd recommend using area portals dividing exposed sections so your map is optimised.

Thank You. The one im trying to get as a skybox is the blue that graudally fades to red on the bottom. Im not sure which one is right, so i will try both suggestions.
Thanks for the help.

For the blue to red with little buildings in the background I think it's the skybox texture, and then setting the correct skybox. But I don't know the name of the correct skybox for you.