Domain authority is a metric developed by leaders of SEO, the one and only Moz and the bigger is your DA the higher is your rank and traffic. Probably you've seen DA as 100, don't get upset if you don't reach it, only sites like Google and Facebook have DA 100.
a. The most important factor is your link profile, more backlinks pointing to your website means higher domain authority, so you must do a detailed backlinks research and get rid of bad backlinks(upon your request backlinks will be removed if that webmaster not respond then you make use of disavow tool) and next step is to focus on gaining good backlinks (hard to be done with so-called content marketing, if not work for you then it remains hiring SEO company).
b. Internal linking must be done, it seems this improve domain authority too, basically spiders will visit those pages as well and take a note if linking is done with a keyword.
c. Update your website with fresh content, daily if possible, this will improve your domain authority as well.
