how to get back link for payroll software related websites?

You need to find the same niche... if you manage to find web sites which you can link to in the same niche then it is awesome, otherwise, you can do on other niches as well.

please also share with us ..

Rather then just focusing on such a small niche you should get links on all business related websites. Create a demo version and submit it to various software downloads sites which will include link to your site. You can then contact business blogs and ask them to review your software with a link to your site.

if they have PDF file you can get a link on that but make sure its crawl-able.