BulgarianPod101 gives you audio and video lessons to get started with Bulgarian.
This site provides a list of podcasts in Bulgarian.
Clozemaster provides sentences where you fill in the vocabulary to help you learn in context.
LingQ offers lots of content allowing you to immerse yourself in the language.
Lyrics Translate is a site that provide community translations for songs.
A useful grammar guide for learning Bulgarian.
Great way to practice your Serbian with other learners.
iTalki provides online classes with native speakers.