This game, admittedly. I wasn't that active on the Internet until about 4 years ago, I had to catch up on 10 or so years of Videos, Memes, Webcomics...

Its hard to remember but I think I remember how:
I was watching somebody on youtube playing SCP-containment breach but I wasn't interested, but my curiosity got the better of me, so I watched more play the game to find its popular! I was hooked into SCP and went to find out that you can read all about the SCP's on the foundation! and I got hooked into it.

And that my friends, is how I found out about SCP, By youtube

I'm not even sure anymore. I think I'd first found out about the SCP Foundation by googling something about the black plague and eventually found SCP-049.
This resulted in me reading for hours on end until I'd gone through pretty much every SCP that existed back then. From there on I eventually stumbled upon the SCP games (087, 087-B and eventually Containment Breach)

So all in all, the good doctor lured me here.

I play a game called MUGEN (Google is your ally), and one of the characters I could download was SCP-173. I had no clue what it was. Later that day, I found a Minecraft mod, and it had SCP-173. So, I finally looked up SCP-173, and found SCP:CB videos belonging to Markiplier. After watching that, I downloaded it (It was v.0.6.5 at the time), and joined the forums. A short while in the stay at the forums, I found out that Regalis did not make these up, a whole group did (The SCP Foundation gaiz). So yeh.

Peanutbuttergamer
http://www.youtube.com/watch?v=RF7NCiRzgAc

I started by watching some gameplay of SCP:CB made by Markiplier (the videos were from 2012...), I initially though that all the SCPs were made by the game authors (I didn't know that the only developer was Regalis...) and I liked them all (the ones in the game). Then when he found SCP-682's document he said the famous phrase: "Man, if anybody READS THESE SCPs, you'v gotta READ this one! GO TO THE WEBSITE and find it out, it is AN INCREDIBLE READ." in it; I was confused and I decided to see why he said that...
I searched "SCP-682" with I.E. and I really enjoyed reading the article about him (and I discovered that the SCPs had their own website and that there were more of them other than the ones game...), so I decided to see the pages of the other SCPs that he found in the game and then I started to read some of them on my own: that's when everything really started.
Today, I'm stilly happy to know that I decided (for some lucky reasons...) to check SCP-682's page that day (it was in July...); that leaded me to the game's forum where I'm writing today. Probably It seems like I'm exxagerating, but knowing about the SCP Foundation, with over than 2000 SCPs to read, gave me something to do when I was at home in Summer: both a GAME and a new ultra-interesting WIKI! (I'm a bit lonely, so having a new Wiki with a lot of interesting stories to read and a new game was FANTASTIC! Since I spend the most of the by staying on the PC...)

You can see the video here:

how did i discover it? god i dont fucking know man, markiplier probly, but shit

I was watching Markiplier play Containment Breach, and he mentioned Incident-096-1-A. That prompted me to look it up, and I discovered the Foundation Wiki. Up until then, I didn't realise that all of the SCP stuff came from there.

Ahhh the joy of Garry's mod. I was looking through my addons to what was new and I came across something that looked like a strange alien shaped chicken. I downloaded it and then went in game. It scared the living crud out of me so I decided to look up the game. I found the foundation a few days later.

TVTropes, which then led me to containment breach which then led me to where I am today.