If you downloaded Serbian 101 e-book, you can easily access the video lessons explaining how to use the verb TO BE and practice both writing and speaking with it:
Uvod = Introduction
Watch the videos and try to pronounce the personal pronouns and the verb TO BE together.
Video with explanations
To Be - Classroom Practice
Online Exercise 1
Online Exercise 2
Let me remind you that the previous beginner lessons can be found under Serbian for beginners tag, as well as on SerbianLesson pages.