Everything starts with an idea, but it can be difficult to move past that stage. Your designer will help you refine your idea, but it is helpful to ask yourself a few questions before you bring it to the table. Ultimately how well prepared you are will be what sets the tone for designer/client communication during the project.
What do you want your design to accomplish?
Are you having a logo designed to represent your business? Are you having your existing website redesigned? Need business cards? What do you want the design to do for you? That may sound like a silly question on the surface, but answering it can help you get more specific with your design needs.
What do you want your design to say?
Is there a mood you are attempting to set? Do you want to come across as orderly and professional, or relaxed and casual? Again, your designer will probably ask you all of these questions but knowing the answers to them ahead of time will allow you to answer them with confidence rather than feeling on the spot, and this will ensure excellent designer/client communication throughout your working relationship.