Bulgarian is spoken by about 9 million people, mostly in Bulgaria, of course, but also in nearby regions such as Moldova and Turkey. It is mutually intelligible with Macedonian and closely related to other South Slavic languages like Bosnian, Croatian, Serbian and Slovenian. It is more distantly related to Russian, Polish, Czeck and other Slavic languages.
Old Bulgarian was also known as Old Church Slavonic and was a major literary language of Europe in the Middle Ages. Modern Bulgarian draws mostly on that history (and also from literary Russian) and is written in the Cyrillic alphabet. It has some grammar peculiarities which differentiate it from the other Slavic languages and has a reputation as being a difficult language to learn. Unless you know one of those related languages, you won't find much familiar ground. Bulgarian also borrows from Turkish and some international vocabulary from French, English, Latin and Greek. There aren't many published commercial products to learn Bulgarian.
FSI Language Courses
The single best online resource to learn Bulgarian comes from the old FSI language materials made by the US government. There is a 2 volume set with texts (in pdf format) and audio (in mp3 format) that is rather old but very comprehensive. The best part is the price - you can download this course in its entirety for free!
Transparent Language offers a free vocabulary trainer to get started learning Bulgarian. Get your Free BYKI™ Lite download by Transparent Language