Ideally you have a friend to answer questions, but I've found the best thing to give new players is an understanding of the basics. You don't need to start off understanding every hero, ability, and item but you should know how the enemy AI behaves and the actual conditions of the game. In my experience, every new player comes in and is told a bunch of stuff about heroes and items, but nothing about how there is a miss chance if you attack from low ground to high ground, how creeps spawn at every 30 seconds, how the jungle creeps spawn at every minute and how you can manipulate their spawning, the runes spawn in one of two locations every 2 minutes, etc.
Probably the biggest thing that new players never get told is how last hitting, denying, and pushing works. They will know that you only get gold for the last hit, but be unfamiliar with the concept of NOT pushing the enemy tower especially in the beginning. They will mindlessly try for last hits and wind up under the enemy tower where their opponents will have a much easier time getting gold and experience for the first 10 minutes, creating a horrible snowball.
In the end, you'll try to explain this and their response will be, "STFU and play". Then you lose the game 20 minutes later.
