Before you start, you need to list the basics:

What is the product or service being promoted
Who is the intended audience (age, gender, where do they live)
What purpose will be addressed
What response are you after
What promotions have been done for this product or service previously
Are there any existing materials or should new ones be created (logos, images, slogans, copy)
How will the information be distributed (brochures, direct mail, magazine ads, person to person, TV, radio, etc.)
When do you need it finished (be realistic and allow as much lead time as possible)
Do you have a budget to consider

When you have your list and any existing materials assembled, you are ready to call your creative service to set a preliminary meeting. If you do not have a good working relationship with a creative service already, call and ask to see a portfolio.