Instructions
Download and install WireMod. It can be found on the moddb.com website.
Look through WireMod tutorials and find one you'd like to create in your game. Tutorials can be found all over the place, but if you're having trouble, check Ducky's Tutorials. Write down the website address.
Start "Garry's Mod" from your Steam Library. Click "Start New Game" and choose any map from the list given to you.
Press "Shift+Tab" to bring up the "Steam Overlay." Near the bottom of the screen, you'll find "Web Browser." Click the button and type in your tutorial's Web address into the window that pops up.
Watch the tutorial. After every step, pause it and press "Shift+Tab" to return to your "Garry's Mod" game. Reproduce the steps shown to you in the video and, in no time, you'll have your own WireMod creation.