| Ask your teacher.
No teacher?
Get a teacher.
This is not something that can be done on the internet. Seriously.
It could be:
not enough rosin
too much rosin
not enough pressure
too much pressure
poor grip
loose grip
tight grip
bad strings
bad bow
bad hair
bad angle
bad weather
bad left hand hand
bad right hand
.....
etc
I'm not trying to be a wiseacre -- this requires someone actually watching you and hearing you and making their own determination and working with you to solve the problem.
In the meantime, make sure to play
L O N G T O N E S
up and down bow, slow, with the metronome
try to use the weight of your arm, no more, no less, to 'push' on the string. try to keep the bow perpendicular to the strings. start with open strings until you get a clean sound, then work on the left hand. | 
