Recommendations

Make sure the timer is suited to the motor's capacity.
Do not buy an all-purpose timer model. The timer must have a minimum capacity of 15 amps and be designed to control motors. An appropriate model can also be used in winter for your vehicle block heater and for outdoor Christmas lights (or other lights).
If you heat your pool with a heat pump, then the use of a timer is not recommended. This is because the water must be kept circulating the entire time the heat pump is in operation.
Make sure your pool water is clean and properly balanced chemically (pH and chlorine) at all times.
If you use a chlorinator, you will have to readjust its flow.
Did you install your timer outside? Make sure you buy a waterproof model.
Timers for in-ground pools are more expensive and must be installed by a master electrician. If the pump is located above the water level, make sure there's no air leakage and that the pump doesn't dry up. Otherwise, it could be damaged.