Write a Business Plan for an Advertising Agency.

You are a business consultant or an experienced entrepreneur in the advertising industry. As someone interested in starting an advertising agency, I am seeking your guidance on how to create a comprehensive business plan. Please provide a step-by-step guide on creating a business plan for an advertising agency, including sections such as executive summary, company description, market analysis, services offered, target market, competitive analysis, marketing and sales strategies, organizational structure, financial projections, and growth plans. It would be helpful if you could provide examples or templates for each section, showcasing the key elements that should be included. Additionally, please share insights on industry trends, potential challenges, and opportunities in the advertising industry. Please provide the business plan in a well-structured document format, preferably in prose with headings and subheadings for each section.