You don't have to be a philosophy major to talk to atheists about God. Here are three simple arguments to help you in your task. Practice them on a friend before you try to use them in real life.
Warning: These are "negative arguments," in that they do not prove the existence of God directly, but they do reveal that only the existence of God can account for the objections each raises.
1) Moral Argument: How can there be good and evil or right and wrong? Another way to put it, if we're playing a game then someone has to create the rules. If there are no objective rules then there are no rules. All human acts would be morally arbitrary and neutral.
Was Hitler evil? By whose standards? Does the atheist have a good answer to this?
So then, there must be an objective lawgiver - and this we know as "God."
2) Cause and Effect Argument: This is also referred to as the cosmological argument. We observe cause and effect in the world. Yet it is absurd that the chain of cause and effect would go back in an infinite directly. There must have been a first cause. This first cause we know as "God."
Think of one million people leaning on one another at an equal angle. At the end of the leaning line of people, there must be a wall to hold up the first person. If there weren't something supporting the leaning line, everyone would just fall over.
3) Design Argument: This is commonly called the teleological argument or "watchmaker" argument. There is order and design in the universe (e.g. mathematical ratios observed in sea shells and bee hives). Since there is a rational order in the universe, then there must be a rational mind behind. To put it another way, if there is design, then there must be a designer.
The common version of this argument is that if you were walking in the desert and you came across a gold watch, you would not assume that the gold, steel, springs, leather strap, crystal, etc. randomly came together to form a sophisticated machine. Rather, you would assume that the watch was made by a watchmaker. So then, the world is ordered and more sophisticated than a watch, therefore there is a supernal "watchmaker," and this we call "God."