The Encyclopedia of Small Business can help you understand the differences between the structures (C Corporation, Partnership, LLC, S Corporation, Sole Proprietorship, Cooperative). NOLO's Small Business Start-Up Kit will help you choose the best one for your plan.
Business Plans Handbook shows sample plans for a variety of businesses. You can search by structure ("s corporation") to see what types of companies choose each.
Research your competitors (or startups in related industries) to see how they've deployed key personnel and their titles.
The Bureau of Labor Statistics allows you to search for this information for different states or metropolitan areas.