So after reading through the novelization, the entire new EU and also the visual dictionary for TFA I do have a personal theory.
Snoke was a Jedi in the order before its demise. He was a general in the Clone Wars and fought alongside all the greats, even those like Anakin Skywalker, Yoda and Obi-Wan Kenobi. He was a great Jedi, but when Order 66 happens he flees. My personal theory is that he was on Corruscant and maybe found out who Sidious really was and realized that there was nothing he could do. So he flees to the Unknown Region.
Now it seems in Aftermath that Palpatine later on during his reign started to look at the Unknown Region, his advisor from ROTJ said in Aftermath that the purity of the Dark Side was there. I think Snoke ran to the unknown regions just to get out of the grip of the Empire. However, he accidently stumbles upon something, a temple, a relic, maybe even predating the Sith., just flowing with the Dark Side. Slowly Snoke becomes corrupted by the Dark Side and even more powerful in learning from this ancient temple/place.
Snoke starts to find some followers; they call themselves the Acolytes of the Beyond, which were mentioned in the book Aftermath as dark side fanatics. After the fall of the Empire Snoke sees a weak galaxy and wants to take control. So he seeks out Luke Skywalker. He learns that Luke is creating a new Jedi Order. He figures his plan two-fold. 1. Corrupt a young powerful force wielder into a puppet. 2. Use the puppet to kill Luke and the rest of the Jedi before they can rise.
Snoke proves to Luke that he is an old Jedi Knight from before the Clone Wars. Luke finds records on him and accepts him knowing he needs all the help he can get, also he needs one Jedi that actually remembers many of the old ways of teaching/running an Order. So Snoke becomes one of the teachers at the new Jedi Order.
So Snoke sets his sights on Ben for many reasons. One his parents don’t pay attention to him, he is of Skywalker blood therefor extremely strong with the Force. Also he has personal issues, but more importantly he can use Ben’s relation to Vader as a tool to seduce him. Slowly Snoke begins to pump up Ben’s ego. The Visual dictionary actually says that Kylo holds the same rank as Vader in the Empire by design. So over the years Snoke turns Ben, and a group of followers calling themselves the Knights of Ren.
The Knights of Ren plan to destroy the Jedi Order and Luke with it. Before that starts to happen Luke learns of Snoke and his true intentions. Maybe Luke learns of the pure evil temple that Snoke was at in the unknown regions. And maybe through a Force dream Yoda tells him he has to go to the first Jedi Temple to learn how to destroy this evil and Snoke. Luke feels there is no time, Luke finds Snoke and Luke and Snoke fight, Snoke is beaten to a pulp. Just as Luke is ready to kill him, he hears screaming back at the Order. The Knights of Ren and Kylo are killing all the students. Snoke turns and runs. But as Luke makes it back the damage is done. Luke decides there is nothing he can do, but to stop Snoke and the Dark Side, he needs to learn more about the Unknown Regions and what they found. So he begrudgingly leaves to go find the answer.
Enter The Force Awakens.
