need to know when touse overdrive

You can just leave your OD on at all times. Though you don't really need it at city speeds, (less than 45 mph). Or you can turn if off if you're struggling going up steep grades on the freeway. Really optional to have it on, though gives you better gas mileage at higher speeds when used.

Keep in mind that when you press the button your turning it OFF not on. Everytime you start the car it will allow itself to engage overdrive. Overdrive is essentially allowing the wheels to turn faster than what the engine is producing creating lower rpms and better mpg. However as Matt said if your driving on hills and the transmission is 'gear hunting' then you may want to consider pushing the button, however do not use at highway speeds for prolonged periods. It can be helpful to turn overdrive OFF when driving in snow or ice to increase natural engine braking, while keeping in mind to use gas even more sparingly as you may lose traction easier.