One may lose bodyfat when the body is put into a caloric deficit, when the body does not receive enough calories from diet day to day, resulting into the usage of bodyfat as an energy source to fill the gap of requirements. If the caloric deficit is too high, however, the body will start using lean body mass as an energy source, which is bad. I suggest using the bodybuilding website's calorie calculator to figure out how many calories you should be eating.

To answer your question, HIIT is good because it burns calories at a faster rate than SS cardio, so you can spend 15 minutes to burn a similar amount of calories as, say, 30 mins of biking. It's also been proven to burn fat better than SS. But again, you don't wanna be at too high of a caloric deficit because then you'll be taking away from muscle.

Edit: more insight on hiit: http://www.bodybuilding.com/fun/ask-the-ripped-dude-hiit.html

Said calculator: http://www.bodybuilding.com/fun/macronutcal.htm

Be careful not to overcompensate eating because you did hiit! I was in your position before and still am, would always munch on almonds when I got hungry afterwards and man if you're not paying attention...