This is a lot easier than it looks! First off, I would recommend playing it on regular where you can die so you get used to the game first. In Hardcore if you die, you lose that character.
1)First off use the wizard because of I felt more comfortable keeping it ranged and he is easy to use to kill really fast.
2)Play on easy! It makes things much easier and it doesn't seem to give you less exp. But dont expect drops to be the same! I didnt find a legendary until halfway through Act 2.
3) Play alone! if you play with other people they can either slow you down or seperate on you and your enemies will be harder. Highly recommended to play alone!
4) What I did was explore everywhere, kill everyone, destroy everything, and pick up all the books you can find for exp. Doing this I got to level 30 on the level Tower of the Damned Level 2 right before the Cydaea in act 3.
5) If you have it use the Legendary Infernal helm! That 25% exp bonus is really nice! If not just put the highest Ruby you have in a socketed helm.
6) Keep you Templar as your follower at all times once you get him! For his ablilites use Heal, Loyalty, Onslaught, and Guardian.Tactics(For Wizard only)
In the beginning use Ray of Frost for bosses/strong minions until you get Disintegrate at lvl 21, Once you get that use it on big groups even because that will hit everyone expecially when you get the rune Convergence. As for your primary attack use Spectral Blade on big groups until you get Disintegrate. The blades are really good when you get backed into a corner especially because you can just start hitting everyone.
For the other skills I used
Defensive/Diamond Skin- Comes in handy when you are under pressure and are being surrounded by enemies, helps keep alive.
Force/Hydra- its always good to have extra things hitting at the enemies, even when you are running away they will attack the enemy, can save your life!
Conjuration/Storm Armor- This one is my favorite other than Disintegrate! Enemies comming near you will get hit by lightning which seems to be stronger the higher your damage! It gets even better when you get the first rune, any attackers melee or ranged will get hit by it if they hit your armor!
Mastery- dont waste your time, both beginning ones suck in my opinion, and aren't worth the arcane to use.
Passive 1/Blur- Reduce melee damage! Very good!
Passive 2/Glass Cannon- Gives 15%, to damage, good news is you don't really have to worry about your resistance in Normal because there are hardly any enemies with abilities like Arcane magic.
Attributes- When it comes to gear make sure you are getting up your intelligence and Vitality, most important, a huge thing to use is gems when they are socketed.