Security Issues to be resolved in my Magento 1 based website

I received below message from webmaster, i want to clean my website and install some security measures so no one can hike in the future, i am using latest version 1.9.3.8 To: Webmaster of [login to view URL], Google has detected that your site has been hacked by a third party who created malicious content on some of your pages. This critical issue utilizes your site’s reputation to show potential visitors unexpected or harmful content on your site or in search results. It also lowers the quality of results for Google Search users. Therefore, we have applied a manual action to your site that will warn users of hacked content when your site appears in search results. To remove this warning, clean up the hacked content, and file a reconsideration request. After we determine that your site no longer has hacked content, we will remove this manual action. Following are one or more example URLs where we found pages that have been compromised. Review them to gain a better sense of where this hacked content appears. The list is not exhaustive. [login to view URL] [login to view URL] [login to view URL]