The first step would be to ensure that the website is configured properly and that all system updates have been applied. A common thing we see here with customers is cross-site contamination, so try and ‘containerise’ multiple websites or use different servers.
As mentioned, keeping your website’s core and plugins up to date is of utmost importance. Those updates not only ensure that your website functions properly but also in most cases include security updates. Often web hosts will do this for you if you are not technically savvy.