When does my pill become effective?

Mary • 21 y/o who doesn’t have anything figured out yet. (We’re working on it).
Hey y'all, I just started taking microgestin FE 1/20 (generic for Loestrin) on the first day of my period, which was three days ago. How long do I have to be on the pill before it's effective as a contraceptive? Also, have any of y'all been on this brand and liked or disliked it? Side effects? I've never been on the pill before 😩