With atheists you need to speak the language of logic and science, and be familiar with the evidence in favor of theism:
(1) the scientific evidence: the Big Bang, thermodynamics, the open universe, and so on. Here’s an explanation of the basics.
(2) the philosophical evidence: particularly the First Cause/Prime Mover, as set forth by Aquinas. Here is an article that provides an overview and also explains how to reconcile evolution.
(3) the human-rights evidence: the concept of inalienable rights (something that atheists unwaveringly support seeing as they don’t want their right to be atheist taken away) presupposes God. If rights come from a man, a group of men, or the state (which in a democracy is just the people) then the source of the rights can take them away. Rights are not inalienable unless they come from a source superior to man, i.e. God. Know the Declaration of Independence cold.
You also need to familiarize yourself with the history of the atheist regimes of the 20th Century. Bottom line: Atheism has utterly failed to produce any sort of good in the world. All officially atheist countries have collapsed (the USSR and its satellite states) or reluctantly embraced theism (China). There are no “secular democracies;” every democracy has God in its founding documents or has otherwise publicly funded or supported a particular religion.
You basically need to prove that theism is reasonable and atheism is not. Based on the above you should have no problem doing so.