It's not uncommon to hear that question during a worship service but many times the identity of Jesus is left out. How can someone come to Jesus if they do not understand exactly who He is? How will they see their need for giving their life to Jesus if they do not understand what He did and why?
It is clear in Scripture that everyone is a sinner. You ask what is a sinner. The answer is simple---one who has transgressed God's law. In Genesis chapter 2, God told Adam and Eve they could eat of any tree in the Garden of Eden but of the tree of the knowledge of good and evil they could not eat. In fact, if they did, they would die. Genesis 3 records a conversation Eve had with the Serpent. Following the persuasion of the Serpent to doubt God, Eve took of the forbidden fruit and gave some to her husband. Both ate. Both sinned. God gave a command not to eat but they ate. Following their act of disobedience, their eyes were opened and they knew they were naked. That day they experienced evil. Up to this point it was only something to be avoided but now it was real to them. They now had guilt and shame and even hid from God, which was something they never had done before.
When we talk about sin we must go back to Genesis and see its origin. When we do this, then we can adequately present what the Bible says regarding it and why God hates it. It is also important to go to the Scriptures to learn the identity of Jesus. He was the God-man who became flesh (John 1:14). He was born of a virgin woman named Mary. He came to this world for one purpose---to save His people from their sins. Understanding who Jesus is is essential to calling a person to follow Him by forsaking their sin.
As you present the Gospel today, spend more time talking about the Savior and the sin that He died for. 2 Corinthians 5:21 says, "For He made Him who knew no sin to be sin for us that we might become the righteousness of God in Him."