They were right to tell you 1/2 cardio and 1/2 lifting on a circuit. Yoga & Pilates are also great as they improve your flexibility and help the mind-body connection. I personally love spin classes. That said, you should do whatever you will stick with.
You need to work your heart, teach your body to process fat and build muscle mass.
those are the three key things to focus on.
Here's a good link to help you begin to understand the big picture for heart rate training and heart monitors:
http://www.duathlon.com/articles/1460
And here is a link to a great chart to help you very specifically use your monitor:
http://www.lifetimefitness.com/heart_rate/
Just click on the link to the pdf file. It has two pages, a chart and an explanation page on how to determine your AT. That's a teaser for you and the answer is in the files!
Good luck!!
I've also found learning how to use a heart rate monitor to be very effective. It gives you a very specific way to measure and monitor your progress.