Not Straight Robot

Hello,
I'm building my first robot and I reached a stage where all the mechanics is finished.
My robot mechanics is depending on two wheels at the back with two motors and It depends on the differential drive to go left or right or go forward. every wheel in the back is attached to a motor and every motor is connected to H brdige and then connected to the MCU.
The problem is the two motors isn't identical, they don't have the same speed, there is a small different in the speeds, So when i want the robot to go forward, It doesn't go fully straight line , there is a small curve to the weaker motor
the curve isn't very sharp as the speeds are not very different but they are different
So can anyone help in this matter?

This always happens, you need some way to monitor the direction and compensate for it, it's a type of negative feedback.

Look into quadrature encoding/quadrature encoders/shaft encoders - here's a few links to get started:

Rotary encoder - Wikipedia, the free encyclopedia
Quadrature Encoder Measurements: How-To Guide - Developer Zone - National Instruments