In your main Steam window click on menu item Help.

Then in this help menu click on System Information

Now a new window pops up.
Klick anywhere in this window.

Without clicking anywhere else, now press CTRL and A on your keyboard at same time.
The window should scroll down if you have done it correctly.
With that you have marked everything in that window.

Without touching anything else, meaning no key presses or clicking,
now press CTRL and C at same time.
With that you have copied the content of that window to your clipboard.

Open where ever you want help from (can be mail, steam forum, or any other forum) and paste your clipboard in there, either via pressing CTRL and V at same time.
Or click in any text field with the right mouse button and choose PASTE from that context menu.

• The picture of the keyboard are under GNU Free Documentation License, Version 1.2 / Creative Commons Attribution-Share Alike 3.0 Unported and take from Wikipedia https://en.wikipedia.org/wiki/File:KB_US-International.svg, see https://en.wikipedia.org/wiki/File:KB_US-International.svg for details
  (I honestly dunno who to contribute this to)
  
• The Icon of this Guide is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic and also from Wipedia / Wikimedia
  see https://commons.wikimedia.org/wiki/File:Intertec_Superbrain.jpg for details
  (Again I am confused who to contribute this to)

http://creativecommons.org/licenses/by-nc-nd/4.0/
This guide to "How to get your System Specifications" in Steam by BiZZ Keryear is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.[creativecommons.org]
If you want use this work you have to link back to here, mention my username, and aren't allowed to add commercials (or earn money in any other way).