I've always founds having a friend "power level" you through the game at a higher difficulty than you playing solo would be able to complete is the best means of leveling. That being said, if you have no one able or willing to do that for you, just play through the game on normal until you find it's becoming quite easy to defeat most enemies. At that point, feel free to go ahead and increase the difficulty to hard, expert, master, or even torment 1 or higher. Once you find some decent gear (yellow gear is better than blue gear, gray and white being the worst, and orange being legendary and the best gear available until you find complete sets which are green), then you'll really be able to amp up the difficulty! Feel free to jump into some online multiplayer with randoms in a PUG (pick-up game) to increase the amount of experience you gain for even faster leveling! Happy gaming! :)