Hii Chandana
...well ....this is natural ..I also used to face such kind of difficulty ..see...suppose u have mass m1 and mass m2 ..and they are tied to the string which passes through a frictionless pulley then ...
the tension in the string is given by T = (2g x m1 x m2 ) / ( m1 + m2 )
the acceleration is given by a = (m2 - m1 ) g / ( m2 + m1 ) ( assuming m2 > m1 )
( if the answer comes negetive ..its all right ...the thing is ..just the supposition of direction is opposite ...your teacher may have a different supposition ....{so take the mod value } )
thrust acting on the pulley ..i.e. reaction at the pulley is given by R = (4 g x m1 x m2 ) / ( m1 + m2 )