If you see a malware or a phishing warning on your website, it means Google has reviewed your website and they found deceptive content on it. In order to protect you and your visitors, they show this warning message on your website.
The first thing you should do if you see this message on your website is to try and find the exact cause. Removing is simply vital to avoid any issues with your hosting account and your visitor's safety.
Once you've done that, you can visit the following link to request the warning to be removed: https://developers.google.com/webmasters/hacked/docs/request_review
Google will review your request and remove the warning once it's confirmed that you've removed the malicious content.
If you are using WordPress on your website, you can use these guides to clean it and secure it: