Everything you need to know about blinker fluid, AKA turn signal fluid:
These days, you can actually perform this task at home, in your driveway or garage. No need to go to a service station. It's a good idea to check with your local auto parts store to be sure you get the right fluid for your car or truck, and to obtain helpful advice if you're unfamiliar with the procedure(s) required to top off your vehicle's blinker/turn signal fluid.
Finally, be aware that newer vehicles may require that you periodically change your blinker fluid. Check your owner's manual to be sure. The following video will help illustrate the steps typically required to perform this important task yourself: