How do I know what to say when telling people that they're living in sin, like my family and friends? I'm scared that whatever I say will not even be listened to and then it's just going to be a squabble afterwards. But I know obviously it is more important to warn them about the life they're living.
People judge the value of what they are being told by who is telling them. A doctor telling you that you have a disease is considered far more seriously that friend down the street even though both may say exactly the same thing. In other words, people don't always judge facts based on truth. Even though, "The thoughts of the righteous are right, but the counsels of the wicked are deceitful" (Proverbs 12:5).
It is hard to warn family and friends when you are young because you start at a disadvantage. You have limited experience so people assume that you don't know what you are talking about. I remember when I was a teen and tried to talk to some about child rearing problems. "Just wait until you get married, then you'll see." Getting married didn't help much. "Just wait until you have kids, then you'll see." I had children, but then I was told, "Just wait until you have teenagers, then you'll see." Now that I've raised fine young men to adulthood, I'm waiting for the next stage of life: "You're too old to understand."
Few people are willing to listen to someone telling them they are wrong. However, many people will consider an insightful point. So instead of giving the conclusion, give the person something to think about. If you have cousin shacking up with a girl, you might note that you plan on saving yourself for marriage to show her how much you respect her. If someone says, "We can't afford to get married," then ask how they manage to afford to live together. Or if friends are teasing about doing a girl, say that you love God too much go against Him and disrespect a girl. None of these are earth shattering, they probably still won't like what you mention, but you can give them something to think about. If you can make it an observation instead of a condemnation of an individual it is more likely to be considered.
It is when a person directly asks you for your advice that you can be more blunt in what you observe and know. But unsolicited advice is rarely welcomed except by those who are strongly motivated to improve themselves. "The way of a fool is right in his own eyes, but he who heeds counsel is wise" (Proverbs 12:15). "Give instruction to a wise man, and he will be still wiser; teach a just man, and he will increase in learning" (Proverbs 9:9).
"Teach me, and I will hold my tongue; Cause me to understand wherein I have erred. How forceful are right words! But what does your arguing prove?" (Job 6:24-25).