Hello NicoRobin00,
Little Snitch should stop any unauthorized outgoing transmission. The difficult part is identifying which ones are legitimate or not.
I wrote a little diagnostic program to help show what malware might be installed. Download EtreCheck from http://www.etrecheck.com, run it, and paste the results here. EtreCheck is perfectly safe to run, does not ask for your password to install, and is signed with my Apple Developer ID.
If malware is installed, EtreCheck will help you remove it, although you may have to supply a password. If you aren’t comfortable with that, just post the EtreCheck report here and other helpers can tell you exactly what files need to be deleted and how to do so.
Disclaimer: Although EtreCheck is free, there are other links on my site that could give me some form of compensation, financial or otherwise.