I maintain a website with lots of information like this:
Lose weight and build muscle with proper diet and exercise
If you are already lean then you can skip the cardio and just do high intensity weight lifting. You should stick to big compound movements and can utilize supersets to cut the time down. Train your entire body 3 times a week. For example.
Mon/Tue/Wen
Squats 2 sets of 10
Lunges 2 sets of 10
Bench Press 2 sets of 10
Deadlift 2 sets of 10
Standing Press 2 sets of 10
Bent Rows 2 sets of 10
This would hit all the major muscle groups and be something you could get done in about 30 minutes. If you can complete 2 sets of 10 then add weight for your next workout to bring it down to failure at 8. This kind of program will typically add about 10 pounds of muscle in about 3 months if you are just starting out. That extra muscle will fill out your body nicely.