how to post others ads in my website? and how to approach companies.?

You can partner with various companies that will provide the ads, such as google adsense. there are others out there.

and how to approach companies.?

you have to build relationships with those target companies. Its a bit more complicated that just picking up the phone and calling someone. If your website has enough traffic or members that they are interested in, they will start calling you.

you can use google adsense services for your website, it is best onlne advertizing company and also a best way of earning.