We recommend you to install the phonetic keyboard layout because most letters are located in the same places where the corresponding sound in English is. Here is how to set up the keyboard layout:
You can also view this video tutorial for Windows Vista:
http://www.youtube.com/watch?v=RKKDo6Fk4k0
Note that on older Windows versions there might not be Bulgarian Phonetic but only the standart Bulgarian layout. In this case you might need to install a software which will add the phonetic layout. There is a variety of such free applications in the internet.