Applegate's interview with "GMA" was taped Monday.
The actress' publicist, Ame Van Iden, announced earlier this month that Applegate was being treated for the disease after it was detected through a doctor-ordered MRI.
"I was so mad," she says in the "GMA" interview when she first heard the news. "I was just shaking and - and then also immediately, I had to go into ... `take-care-of-business-mode,' which was ... I asked them, `What do I do now? What - what is it that I do? I get a doctor, I get a surgeon, I get an oncologist? What do I do?"'
Applegate, 36, says she "immediately made those appointments and immediately called around for ... someone to start teaching me how to live macrobiotically." She was referring to following a healthy diet of fish, grains, beans and vegetables, and avoiding processed foods.
The actress, whose mother battled breast cancer, says she began getting mammograms at the age of 30.
Applegate is scheduled to appear on a one-hour TV special, "Stand Up to Cancer," to be aired on ABC, CBS and NBC on Sept. 5 to raise funds for cancer research.
She has been nominated for an Emmy and a Golden Globe for the ABC show "Samantha Who?", in which she plays a woman who wakes from a coma with no memory of who she is.
(Copyright 2008 by The Associated Press. All Rights Reserved.)
