Body fat percentage and six pack?

So I have my own body fat tester that says I have 19% body fat, but if I measure myself again right after I just did it will sometimes drop to 17.5% or up to 21%, so I don't trust it. I went online and used a bunch of BMI/ body fat caluclators and my results ranged from 7% to 20%. I am 15 years old, 5' 7" weigh 135 and a guy. I'm not conserned with how fit I am, but i want a six pack. My abs have some definition, but I can only see it when a light is shinning at me. Thanks!