Before you hit the weights, start with 5 minutes of HIIT. High Intensity Interval Training. It will kick you into that fat burning zone before you do lifting.
Here's an easy way to do it: Hop on a bike. move the resistance up a few notches. The ride at an slow and easy pace for 45 seconds. Then for the next 15 secs. pedal for all your worth. Repeat this four more times for the 5 min total.