To directly answer your question: Either "a" or "the" could be appropriate.
"A" means it is one of many possible things. In this case, you would be saying, "here is a class that does X". That is, there may be many classes in this program that do some variety of X. This is one of them. Or more likely, there may be many classes in the world that do X. This is one of them. Think of other context. You might say, "Bob is a friend of mine." Of course Bob is one specific person, but he is not your only friend, certainly not the only person in the world who is someone's friend. So when you first mention him, you call him "a friend", not "the friend".
"The" means one specific thing. We use "the" when the specific thing has already been identified, is obvious from context, or is the only one in the world, or at least in the scope of the conversation. In this case, if this is the only class in your program that does X, you could say, "The class that does X". The fact that there are other classes in other programs that also do X is something that you are considering irrevelant.
All that said, for a comment in a program, I normally wouldn't use either. I usually write comments on or within functions as imperatives. For example, "Calculate sales tax." "Send the confirmation email to the customer." For classes, I describe the thing they represent, usually not using complete sentences. "An item that is stored in the warehouse. Does not include items on the shelves in the stores." This is brief and to the point. It's not necessary to say, "I prepared this class to calculate the sales tax." Of course it's a class, the reader can tell that from the "class" declaration. You don't need to say "prepared to do". As opposed to what? I discovered it lying on the ground on my way to work? :-) Program comments are often more like headlines than full sentences. This isn't an essay, just a hint to other programmers who have to read this thing.