Connecting a servo motor to an Arduino
You will have to check in the servos datasheet for the maximal current, that it draws (motors draw more current, when inhibited by mechanical force, so be sure to get the maximal current from the datasheet). From the data, that you gave us, I think it will draw rather much current. So you should not power it over the USB port of your PC (which gives 500mA at maximum). Also you should not use the 5V pin of your Arduino, or you may fry components like the diode, that lies in the current path from USB to 5V pin.

For your step down module, you will also have to check, if it can provide the current, that the servo needs, or you may kill it. If you want to use USB, you can use a standard USB charger or powerbank. Often these can provide more than 1A (depending on the actual product). Most USB wallwart chargers have a current specification on them. But (as said earlier) don't provide the power through the Arduino. Provide it directly from the source, so that the possibly high current for the servo does not have to pass trough the Arduino. You can also power the Arduino with the same source in parallel to the servo, so that you don't need two sources or a converter.

For more info, you will have to provide more information about the servo