Titan Quest

Dream mastery is overpowered. As long as you pick two good mastery spheres, you'll be okay. You can always reset your points, for a cost. The cost maxes out at a level that is pretty manageable at higher levels.

Mages are a lot harder to play than warriors. If you're going to do a mage, you might want a buddy.

This website is really useful if you are having a hard time finding something for a quest or for any general help: http://www.gamebanshee.com/titanquest/.

Popular combinations are Warfare/Dream Mastery for the ultimate melee machine. Or Hunting/Dream Mastery for potentially devastating bow attacks that can decimate entire groups at a time. Hunting/Rogue is also popular for spear wielding.

These are hands down the most powerful classes, but not the only playable ones. I recommend against Dream being your primary mastery, despite it being the most overpowered one. It works best as a backup and you can pretty much attach it to any other mastery and make a powerful class.

I recommend, once you get a certain distance into the game, to download and try out TQ Defiler. It's a very handy program. It's technically a cheat program, but respecs in Titan Quest are absurdly expensive and Defiler makes it easy. It also has a few other handy features, such as character cloning. Clone your character, respec him, experiment without fear of gimping yourself and wasting large amounts of time, and have fun.

Also in the later difficulties, you'll get access to +skill equipment. Horde these, as they're the best modifiers in the game. The reason is that there's a skill cap. You can use +skill gear to get to that cap and then remove excess points and put them into other skills. You can max out a ridiculous number of skills this way.

For a probably easier class to play as, you may want to consider a warrior combo for your first game. They're pretty fun, especially with the rag-doll physics you don't get to see as an archer, and they can also be really damn powerful. Warfare/Dream is the most popular and probably most overpowered class in the game, it can do the highest single-target damage out of any class, and has lots of AoE abilities. But really, warrior combined with a lot of other classes can be strong. Warfare/fire is a nice hybrid class, warfare/earth can give you nice defensive abilities, warfare/rogue can make for a really nice spear oriented class, and so on.

Also it's been said before, but Dream mastery is the most overpowered one in the game. It's abilities can go with literally every single other mastery and any combination with Dream is a good combination, so don't be bashful about picking it.