Motor calibration for Romi 32U4

I assembled a Romi 32u4 kit and am using Arduino IDE to program it. When I program it to go forward it curves to the right. When I assign the motors different speed values it changes every time I power it on or off. Is there any software that I can use to calibrate it to make it go straight consistently?

If the motors have shaft encoders, see this post: 3pi Standard Edition Curving - #2 by Jim_Remington