Although there are things you can do if you're failing a college class -- or even if you've already failed it -- breaking the news to your parents is an entirely different problem.
Chances are, your parents are going to want to see your grades from time to time (translation: every semester), especially if they're paying for your tuition. Consequently, bringing home a nice fat "F" probably wasn't on your list of things to do this semester. Given that no one is going to be happy about the situation, the best approach can be a basic one: be honest, be positive, and be sincere.
Be honest. Be honest about the grade. What is it? A "D"? An "F"? It's better to only have this conversation once. "Mom, I'm going to be getting an 'F' in Organic Chemistry" is way better than "Mom, I think I'm not doing so well in Organic Chemistry" followed a few minutes later by "Well, I've failed most of the exams" followed by "Yeah, I'm pretty sure I'm getting an 'F' but I'm not totally sure -- yet." At this point in your life, you undoubtedly know that parents deal better with getting bad news that can improve later than getting kinda-bad news that gets worse later. So just be honest about your grade. What is it? What part of the equation is your fault (not studying enough, spending too much time socializing, etc.)? Own up to the situation and the responsibility. Honesty may be a bit uncomfortable, but it is undoubtedly the best policy in situations like this.
Be positive. Present the situation as real -- but also as a growth and learning opportunity for you. Okay, so you failed a class. What did you learn? That you need to manage your time better? That you spent too much time just hanging out with people? That you need to take fewer units? That you need to be less involved with clubs? That you need to cut back on your work hours? Let your parents know what you're going to do differently next semester so that this won't happen again. (Because really, who wants to have this conversation again?!) "Mom, I failed Organic Chemistry. Looking back, I think it's because I didn't spend enough time in the lab/didn't balance my time well/was too distracted by all the fun things going on on campus, so next semester I'm planning on joining a study group/using a better time management system/cutting down on my cocurricular involvement."
Additionally, let your parents know what your options are in as positive a light as possible. They most likely will want to know, "What does this mean?" Are you on academic probation? Able to keep up with your other courses? Not able to stay in your major? Be prepared with how you can move forward. Let your parents know what your academic situation is. Talk to your adviser about what your options are. "Mom, I failed Organic Chemistry, but I talked to my adviser since I knew I was struggling. Our plan is to have me try it one more time next semester, when it's offered, but this time I'll join a study group and go to the tutoring center at least once a week."
Be sincere. You may think you're a pretty good liar, but parents can smell dishonesty from a mile away. You know it, and they know. So be sincere about what you're saying to them. Did you just goof up and learn a lesson about how important it is to go to class? Then tell them that instead of trying to blame it on a bad professor or lab partner. Also, be sincere about where you're going from here. If you don't know, that's okay, too -- as long as you're exploring your options. Conversely, be sincere when you listen to what they have to say. They aren't likely to be happy about your failed class, but they probably have your best interest at heart.