To make them less angry
Your parents aren't your enemies; "they only want what's best for you"
You should tell them when they are feeling neither bad nor good. If you tell them after you have done something good for them, you ruin their spirits for the day, and possibly the way they view you when you help, and if you tell them on a bad day, well, you get the picture.
Just tell them the truth and that you want to do better and will try your very best to improve
Show them asap, and just be straight-up honest. Tell them you were having major trouble and should have tried getting extra help. Apologize. That's the best you can do, really. You could also say you can do chores around the house or something to make up for your bad grades. If you own up to your mistakes, your parents will realize you learned a lesson and hopefully won't be too angry.
Tell them you're pregnant. Then when they start freaking out, say you were just kidding, and when they are happy you aren't pregnant, saY you did bad on you're report card. More than likely they'll just be happy ur not prego haha