The kind of content you create depends on your audience's needs at different stages in the buyer's journey. You should start by creating buyer personas (use these free templates, or try makemypersona.com) to identify what your audience's goals and challenges are in relation to your business. On a basic level, your online content should aim to help them meet these goals, and overcome their challenges.
Internet marketing is the job of creating a convincing message that attracts customers to an online product. Successful online marketers come from a variety of backgrounds, including business, graphic design, writing and IT fields. You can find resources on the web to learn more about Internet marketing. Try taking on a small initial project and add the results to your personal work portfolio. Use the portfolio to interview for jobs in the field, or to find work as a freelancer.