Look for a topically relevant group to join on sites like Facebook and LinkedIn where your audience already exists. You should also do a Google search for terms like "your niche + forum" and "your niche + online groups" to see what kinds of community-run sites already exist.
For example, if you're starting a blog about photography, you could consider joining Facebook groups like Nikon Digital Camera & Photo Enthusiasts (26,000+ members) and Nikon UK Photography (13,000+ members).
If your blog is relevant to anyone with a DSLR camera, you're sure to find enough people who'd be interested in reading your posts within those groups.