Make Your Case
Building a case starts with three things:
Make a claim.
Give reasons for it.
Support those reasons with evidence.
For example, you might claim that God exists. The next question is: Why should anyone believe that?
You could point to the beginning of the universe, the design of the universe, or the existence of objective morality. Each of these can provide a reason to believe in God.
Be clear about what your argument actually shows. An argument that points to a creator does not automatically prove every belief in Christianity. It is one piece of a larger case.
Do not try to prove everything at once. Begin with a clear claim and build from there.