Hi-
Do you have any security plugins installed and enabled? I personally like WordFence.
Here’s how I would go about cleaning up:
-Make sure you change your password to something really strong (uppercase/lowercase/number/character)
-Make a backup of your wp-content/themes/ folder
-Write down the plugins you have activated
-Download a fresh copy of the latest version of WP
-Using FTP, log into your server and delete all the WP core files and re-upload the fresh copies (including downloading all new plugin files and reuploading)
-Take a look at your home directory and make sure there are no “odd” files (you can compare what’s in the WP download folders with the folders on your server — if you find any discrepancies, proceed cautiously before deleting)
-Get new Salt keys (good article here: https://www.wpbeginner.com/beginners-guide/what-why-and-hows-of-wordpress-security-keys/)
That’s just a starting point. Depending on your level of technical skill, there are more advanced things you can do. But doing the above is a bare minimum.