There is only one Horadrim Sage, schooled in the most arcane history and lore

The Cairn Stones: Once you beat Blood Raven, talk ROM Gold to Akara to start the longest quest in the first Act. You should be around levels 7-8. Again, Akara has some things to say.

"It is clear that we are facing an Evil difficult to comprehend, let alone combat. There is only one Horadrim Sage, schooled in the most arcane history and lore, who could help us... his name is Deckard Cain. You must travel to Tristram. I pray that he still lives."

First what you have to do is find the Tree of Inifuss and get the Scroll of Inifuss from it. It is located in the Dark Wood, which is past the Cold Plains, past the Stony Field and through the Underground Passage.

Make note of where there are five pillars in Stony Field, because when you leave and come back to them they won't be on the map. Whenever you think that the monsters are killing you easily and you aren't killing them in one hit or two hits, you should go back and train yourself until you can. 

When you get close to the Tree of Inifuss, there will be a unique monster called Treehead Wood fist, and he is extremely strong and fast. He is a gargantuan beast.