Two very different questions. For how to blog, here are two articles that provide foundations to get you started:
To get companies to blog and share your site, you want to explore something called "blogger outreach". There are plenty of really good, large guides online to help you do this.
Great question and I think blogging is definitely a competitive advantage for stores (over time).
If you can, have a goal to blog ~2-4x per month - this will also create a topic that you can put in your newsletter each week or every second week. I find that longer form content (2000+ words) will perform better longer term.
You can also create list posts and include other companies and then tweet them and they will usually share it.
For you, some ideas could include:
Joshua has included some good links above!