I find that this covers almost everything:
You need a Creative Brief.
The graphic designer you approach should have one to give to you. Here are a number of topics to consider, though this is for large projects and some of the subjects may not apply for you:
Who are you and what do we need to know about you?
Give some background information on how this project came about. What have you done in the past? Give a brief explanation of what you need from the graphic designer.
What are the current trends and challenges you're facing in your industry/ies? What are your competitors doing? Provide information that you feel is relevant such as articles, reports or statistics.
What is the desired end result? What do you want the target audience to do? Try out the product? Understand it better? Increase awareness of your brand? Try to include some SMART (specific, measurable, achievable, realistic and time-constrained) targets as well, for example:
'Gain 500 Facebook likes by August' or 'Increase footfall by 5% this quarter'
Who is the intended target audience for this communication? Can you imagine the type of person you are trying to reach out to? How do they feel towards the market? What are their attitudes towards the product/service? How should they perceive your brand?
What is so great about the product/service? What is the importance? What is the most compelling and persuasive message that will get them to do what you want? Try to keep it as simple as possible and benefit-led.
Benefits and Support for the Proposition
How can you prove the proposition is true? (if applicable) Provide a short list of benefits that support the proposition, directly and indirectly.
Is there anything else that will prompt your target audience to act? If so, what? and how important is it..
What's the first thing you want your intended audience to do when viewing your website? Give you a call? Buy your product/service? Book an appointment?
Tone of Voice
The mood of the design, how it should feel to the audience. Ideally you should try to think of an analogy that has an identifiable personality consistent with what you want, a famous person, a car, a brand, whatever fits.
What are the most important aspects of your brand? What are your brand values, vision, character and personality?
What are the required outcomes from this project? A one page template? A .PSD? A fully working website?
What MUST be included? Your logo, strapline, any legal information, T's & C's? What constraints must be adhered to? Time? Budget? Colours? Formats?
Include any references, websites, info, guidelines, previously designed materials etc.