Everything that has already been said is spot on. Social media is your best friend when it comes to developing your online presence and influence. It definitely starts with your network. They already trust you (or are at least more predisposed to trust you because you already have established rapport with them), so start by sharing quality content with them and grow from there. As you continue doing that, you'll be able to identify what the needs or wants of your audience are and cater your content towards that.
A very big thing that I would recommend is to identify your "why" (see Simon Sinek's TED Talk for more on this). Think about why you are sharing those ideas and contributions with your followers. Be genuine in what you are offering online--care deeply about it, convey how much you care about it, and you'll attract followers who care just as much as you do.