How do you make an arduino sense a 90 degree turn?

Projects General Guidance
1 
Hi I need help using the Ardunio. Again my main subject is 
(How do you make an Arduino sense a 90 degree turn?)

Note I have a "ARDUNIO UNO".

Any tips or tutorial? If yes then ok. 
Thanks!
2

Try to use a magnetometer like the HMC5883L compass module.