The standard way to inflate a basketball is with an air pump and needle. Get a pump similar to those used for bike tires. Add a special needle for ball inflation.
Lock the needle into place using the handle at the end of the pump. Insert the needle into the inflation hole on the basketball. Pump several times.
Since no official inflation measurement exists, test the ball after it feels firm. If it bounces to the desirable level, stop. If not, pump some more and retest.