We use the present continuous tense, aka present progressive (= садашње трајно време):
· to say that something is happening now, at the moment of speaking
o I am sitting.
· to say that something is in progress
o She is studying law.
· for fixed arrangements
o He is having a meeting this afternoon.
· for annoying, repeated actions
o He is always picking his nose.
Now, watch this video for a more visual explanation: