If you're an Internet marketer, then you need to work on one single skill above all others - writing. Even if you outsource your content creation to a professional freelancer, it helps to know what good writing for the 'net equates to, or you won't be able to discern good deliverables from bad ones.
There are different types of content that you'll need, including:
Sales copy
Blog posts
Email autoresponders
Articles for directories
Viral reports
eBooks
All of these require a certain kind of writing that's casual in nature and easy to digest for the prospect, but you also want it to have your personal style and voice so that whenever anyone reads something online by you, they associate it with your brand.