using camera flash

Hello, I have a nikon SB800 and I wonder if anyone as the time/inclination to give me practical help so that I can get started with it. Feeling enthused after Tony's talk yesterday but fustrated as I don't seem to be able to control the amount of light. I must be doing something wrong when I try to use the flash to avoid increasing the ISO. I bought the flash second hand and I think it should be quite clever, although it might not be fully compatible with the D500 if used off camera, it should work on camera and it would be a start if I could crack that bit.