How do people know the gospel of salvation? To put it in its simplest form, someone communicated it to them, whether vocally or written. The gospel of salvation was communicated multiple ways, through the old testament prophecies, through the written record, and through the prophecies of the One who died to save all mankind from sin, and finally through the crucifixion of Christ. Moments after a person comes to the saving knowledge of Christ, they'll start to communicate that same knowledge to others. I know a little girl who got saved when she was about 5 and it didn't take long before she started telling her little brother. The Bible commands believers to tell the gospel of salvation to others.
The means of salvation is faith in the finished work of Christ, His death, burial, resurrection, and ascension. This is the very basis of what the believer is commanded to communicate and the only message we can communicate to others before anything else. There is a tendency in us to start to communicate other truths from God's Word to an unbeliever, before they actually receive Christ. Certainly, if they ask questions, we need to be ready and prepared with answers, but it all needs to be brought back to the basis of fait in the finished work of Christ. In the gospel of John alone, there are approximately 85 times that the word "believe" is used. And, of course, that gospel is written so "that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name."
Jesus said in John 12 "I am come a light into the world, that whosoever believeth on me should not abide in darkness." The only way of true salvation is to believe in the finished work of Christ. Believe that he died for your sins and be saved. John 17:3 "And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent."