Like mentioned before I'll second it. Gundrak and DTK are both great places to start heroics and I'll even throw in Utgarde Keep and Halls of Lightning.

In Gundrak watch out for the Collusus- he's easy but tricky to master for a new boxer.

In DTK watch the Soul mages in the start that toss down the shadow damage circles. That's probably the biggest danger in the whole place lol.

Anyway more detailed guides to specific dungeons are in the dungeons forum if you find yourself stuck on a boss.