(1) define the terms:
Define what you mean by the term "God" and ask the atheist whether they find it (A) more probable that God does not exist or (B) whether they are undecided on whether God's existence is probable or improbable. I suggest framing the issue on whether consciousness is fundamental or whether the material world is fundamental.
If the atheist identified as (A), then they need to rationally justify their position using some form of evidence. If the atheist identified as (B) then the atheist is purely impartial on whether God exists or not and needs no evidence because they have no position on the matter. If the evidence leans in favor of God's existence this would rationally compel them to become theists.
(2) agree on a threshold for rationally warranted belief. Some atheists will demand "incontrovertible" evidence of God before believing. A perfectly rational person, however, would accept that a claim is true if there's more information indicating that it's true rather than untrue.
(3) Present your evidence. This is really where the theist should shine. There are cosmological, axiological, teleological, and ontological arguments in support of God's existence. The theists case will consist of arguments on offense and the atheist will be playing defense. Even if the atheist is successful in negating all of these arguments, they still would have no positive case that God does not exist until they present arguments in support of that position.
The theist is loaded with ammo but the atheist has a shield. The outcome of the debate will depend on how familiar you are with the available body of evidence and how rational your interpretation of the evidence is.