Usually the lower cc engine bikes are lighter (but not always). Cruisers and Tourers are heavier than sport/street bikes most of the time. Dual Purpose bikes are pretty light, but not very sporty on the street.
Oh, and bikes without fairings (body case) are also lighter.
I'd say a good sporty beginner bike that won't kill you and is very light would be the Honda CBR125R (125 cc), its wet weight is under 300 lbs. (avg. is 375 on lower cc sport bikes to nearly 500 on high displacement sport bikes). Also comes with fairing to make it look nice, but be prepared for fairing damage when you drop your bike.
Another nice beginner sport bike is the Ninja 250R (250 cc), and ~375 lbs.
