Depends on what type of business you are starting, but internet marketing these days is definitely a must.
Look into SEO(Search Engine Optimization), Google AdWords, Internet Marketing(as a whole)...Everything related to this topic would be good.
Management and legal skills are also important, though my focus is in marketing. there is a website called LegalZoom.com that, for a fee, will sort out all of your legal requirements.
Back to marketing, you may...