Getting the word out
As JT Smith mentioned, if you'd like to build a business, you need to build a blog where you give free advice, so people start to follow you.
I trust you have some kind of expertise. Or maybe something that interests you that you are currently learning. That can be another great subject to write about. (I do it all the time.)
So the questions then (with quick answers):
How do you get people to your site in the first place?
Slowly. One-by-one.
How do you keep them coming back?
You create content that is interesting for your niche.
You ask questions, that you answer in later posts. (like in this post.)
When to start blogging?
Now.
(e.g using Movable Type or Blio.)