Pay attention to what you mean to be saying when you use a or the to designate something. In general, a indicates one of many possibles while the is restrictive to one specific person, place, event, item, etc. For example, saying, “He picked up a book,” implies that there were several to choose from. One might even think it was a random act. If “He picked up the book,” the implications are quite different. It isn’t just any old book. So consider what you mean when you choose a car over the car, the blue velvet dress rather than a blue velvet dress.