Depends, are you talking about ringtone volume? If so, you can do this through Menu/Settings/Call settings.
If you're talking about voice volume, usually, the Vol + & - toggle on the side of the phone.
I will assume you have this toggle? I don't have a Samsung Galaxy, but usually, they're all located on the right or left side of the phone, but I believe it will be on the left side (not on the front face)
Hope it helps!