Blogs are about conversations. While most people conceive blogging as writing your own blog, it's very much about reading others' blogs and responding.
Google the area of your interest; you'll probably find blogs about it. Then follow Darren Rowse's tips on engaging with other bloggers. Read his post.
- Highlight what they said well.
- Show what they missed.
- Answer questions.
- Summarise what others are writing.
- Show how it applies to you/your students.
- Extrapolate to the future.
- Reflect on the past.
- Build on ideas.
- Be the opposition.
- Ask: what if?
- Be the Devil's Advocate.
Through the conversations you develop, you'll find that over time, you've built a community which can be invaluable to you.