How can I make companies come post on my site? (read desc)

You can’t. All you can do is steal from other sites, like most job hunt sites do. One way to get job openings is to go directly to the company and look at their job openings, then add the list to your site.

As a job posting site your goal is not just to list jobs but to put the right people in contact. If you want companies to post you need to have an audience of talented job seeking individuals.

One way to tackle this is to focus on a specific slice of the job market. For example try and create the best place to find programmers specializing in mobile development. Then try and build a user base of talented mobile developers. If you can tap this group of people then companies looking for mobile web development will come your way.

It’s not going to be easy, this is a crowded space. Focus on something that differentiates you from the others.

@cawlin great answer, thanks.