I want to add intervals to my cycling workouts ~ what's the best way for a beginner to start with intervals? Speed? Going faster maybe a set speed I'm shooting for, for the intervals? or Intensity, maybe using a tougher gear to make it harder to go?

Also what's the best time frame for a beginner to start with ~ 4 min/4 min? or more or less?

I'd like to get clear on this so I can do this tomorrow and none of the websites I've seen have really helped me figure it out. Any help would be much appreciated.