Toriod, how many turns of wire to get 100 uH?
Without knowing the characteristics of your core, you're going to be stuck with cut and try.
If you measure the inductance with, say, 10 turns and it is x uH then you will wind n turns where
n = \$10\cdot \sqrt{x/100}\$
Of course that tells you nothing about the possible saturation of the core, and at 15A that will likely be a significant problem with that small a core size and 100uH (by my guess). Compare commercial cores with known material characteristics to get an idea.
Or, better yet, design your inductor with a known material and geometry and go buy that core before you waste too much time guessing.