Looks like you've been hacked.
Unfortunately there is no easy answer for how to fix this. You will need to clean your site which will involve manual steps to weed out the files which are causing this issue.
I would suggest starting with updating all plugins and themes and also wordpress to latest versions.
This will also help:
https://codex.wordpress.org/FAQ_My_site_was_hacked
You should only get plugins or themes from trusted sources. Never use "premium" products which you found somewhere online for free.