For a full, very thorough argument, I suggest this book. But to simplify, you first have to define what you mean by God. Once you do this, examine whether this God's existence lines up with what you can observe about reality and what you would expect reality to be like if that God existed, and also examine the definition for internal consistency. Every standard definition of God is internally contradictory and almost always inconsistent with the universe we can observe. There is no good argument for God's existence, and there are a lot of good, solid arguments against the existence of the Christian gods.