Reading the following articles will give you a good introduction to WordPress security:
YOAST: WP Security in a Few Easy Steps:
https://yoast.com/wordpress-security/#wpsecuritykeys
WP-CONFIG Security Keys ( WP Beginner )
https://www.wpbeginner.com/beginners-guide/what-why-and-hows-of-wordpress-security-keys/
HOW TO REMOVE GOOGLE BLACKLIST WARNING ( Succuri )
https://sucuri.net/guides/how-to-remove-google-blacklist-warning
CODEX: HARDENING WORDPRESS
https://codex.wordpress.org/Hardening_WordPress
ULTIMATE GUIDE TO WORDPRESS SECURITY (wpmudev)
https://premium.wpmudev.org/blog/ultimate-guide-wordpress-security/?utm_expid=3606929-108.O6f5ypXuTg-XPCV9sY1yrw.0&utm_referrer=https%3A%2F%2Fwww.google.ca%2F