Making a differential robot drive straight is not easy.
I've been building my own navbot for the last few months. Only now is it able to navigate accurately using wheel encoders and dead reckoning.
I first started off making a wall following robot that uses encoder to make it drive straight: WallBot | solderspot
I then started on the NavBot proper: navbot | solderspot
It ends up being very involved.
How much time and effort do you plan to put into your project? How accurate does it need to be?