Problem in understanding bridge rectifier
I totally agree with the comments made about lack of basic research before asking the question.This topic is well covered on the net, in books etc.

Consider a voltage across a diode. For a diode to conduct the anode voltage must be more positive than the cathode.

When a diode conducts there is a forward drop in voltage. This voltage will vary with the current through the diode (often more than 1V) but lets assume a minimum value of 0.7V for a silicon diode.

Voltages are always relative so in a transformer /bridge rectifier circuit we can take our '0V' (ground) from the perspective of the transformer or the load. Normally we take the load. Either way it makes absolutely no difference to the voltage across the diodes and the direction of current.

You always get two diodes that can't conduct because they are reverse biased and two diodes that do conduct giving a 2xVd voltage drop from the transformer to the load. 

The voltage across and current through the load is always in the same direction.