If you preach the gospel often, you will definitely come across those who do not believe in the existence of God. So your first challenge is now to prove to them that God exists before you start talking about Jesus and his sacrifice.
It is very easy.
The fact that a building exists shows that there was a builder. A building cannot exist without a builder. The fact that a cake exists proves that there was a baker. The very existence of a car is proof enough that someone made it.
Scientifically, everything is created by something. It is scientifically impossible for something to exist without being made.
So the very existence of the earth proves that it was made or created.
That trees, birds, animals, me and you exist simply shows that someone made us all.
God is the one behind it all.