When I first started serious weight training I worked each body part once a week w/a 6 day split, one body part per day. That's still my preferred routine but sadly it won't fit my work schedule.
During the last 6 months I've made some really major strength gains, which I attribute to weight gain but still, it's been like nothing I've ever been through before. I didn't just break a plateau, I exploded through it. I increased my weights not once, but twice, from 5-30 lbs, depending on the body part & exercise.
So it's w/a 3 day split, 2 body parts per day. I do 4-5 exercises per body part, 3 sets 8-10. When I find myself doing 10 reps for all 3 sets, I increase the weight on the 3rd set. When I find myself doing 10 reps on this increased 3rd set, I increase the weight for all 3 sets.
Part of this is not letting yourself settle into a comfort zone. Building muscle is not easy--you know, the whole "no pain, no gain" thing. You really have to push yourself.
Then there's the cardio. Initially you might want to limit it to 4 days a week, 30-40 minutes (but NO MORE than 40 minutes). Too much cardio will allegedly burn off muscle mass (I say allegedly b/c I personally haven't experienced this, but if the experts say it's so it must be true, right?).
Good luck!
ETA: I forgot diet! Lots & lots 'o' protein! Probably some carbs too, but I'm still a little leery of carbs. ;-)