You must already have a niche (area of interest)
you have to
- create a market policy
- possible social group
- be member of othe groups of the niche
- make known your blog
- create a product like magnet, offer free and make the users to give their mail
- use the mail in order to write messages, sell products e.t.c.
- be always the student of the niche
- and more
read also
How can I get people to use a free web application I created to help developers like myself track and analyse files and lines of codes in a project? I use it often, and would really like for others to use it too as I think it's very useful.