The Bible actually uses several words that we translate as "sin," but they all have the same meaning: A sin is anything that isn't pleasing to God. It may be something we do or say, or an evil thought or selfish motive -- but whatever it is, it isn't God's will.
Have you ever asked yourself why God doesn't want us to sin? It isn't because He wants to take all the fun out of life and make us miserable -- not at all. Instead, God loves us, and He knows far better than we do what is best for us. He also knows what will hurt us or keep us from true happiness, and when we do those things, we only hurt ourselves. Sin always hurts us -- always.
Some things are clearly not God's will; He has told us about them in the Bible, which is His Word. But what about other situations, such as those you've allowed into your life? Let me ask you some questions. Have you prayed and asked God if these are pleasing to Him? Would you be ashamed or embarrassed if others knew about them? Would they lead a younger person astray? I recall the advice a godly man told me many years ago: "If in doubt -- don't!"
The real issue, however, is this: What place does Jesus Christ have in your life? Make sure of your commitment to Him and make it your goal to follow Him every day. The Bible says, "Pursue righteousness, godliness, faith, love, endurance and gentleness" (1 Timothy 6:11).