To quote Moz (https://moz.com/learn/seo/domain-authority)
"Unlike other SEO metrics, Domain Authority is difficult to influence directly. It is made up of an aggregate of metrics (MozRank, MozTrust, link profile, and more) that each have an impact on this score. This was done intentionally; this metric is meant to approximate how competitive a given site is in Google.com. Since Google takes a lot of factors into account, a metric that tries to calculate it must incorporate a lot of factors, as well.
The best way to influence this metric is to improve your overall SEO. In particular, you should focus on your link profile—which influences MozRank and MozTrust—by getting more links from other well-linked-to pages."
=> basically build links : check https://moz.com/beginners-guide-to-link-building to get you started; having top quality content will certainly help. More usefull resources can be found here: https://moz.com/blog/category/link-building
Dirk