This is a solid question, most of mankind has asked, when told they are to believe in their heart and will inherit eternal life. Others who hear, may only believe for a short while till the issues of life press in and they give in to the world and fall from faith, if continued, they will have eternal life, but unfortunately in hell.
John 14:6 "Jesus said to him, I am the way, and the truth, and the life, no one comes to the Father, but through me". This scripture tells us that God has bridged the gulf that separates us from Him by sending His Son, Jesus Christ! When He died on the cross in our place, He paid the penalty for our sins. Jesus claimed to be God. He didn't leave any other option open. His claim must be either true or false, so it is something that should be given serious consideration.
The fact is that there is written testimony that Jesus existed by both New Testament writers, who saw Him and Jewish historians and others, whereby countless manuscripts have been written. New Testament writer Paul has been documented as writing his letters in the bible 40 years after Jesus was crucified and knew the apostles himself. The best way for you to know Jesus is to ask Him to come into your heart, forgive you of your sins and ask Him to lead your life, when you do this you will find His presence is something you can't deny and you will see personally, evidence of how He changes your life, speaks to you and guides you.