IMHO the best way to learn is by examples.
I would NOT go for a text book.
if you are looking for high school level math, get yourself the Martin Gardner Sci-Am collection. There are 15 of them, most go for under $3 as used (in amazon or abebooks). there's loads and loads of interesting high school level math there - geometry, probability, discrete math, and the list goes on.
if you aim a bit higher, I would recommend Paul J. Nahin books. these require some higher calculus level and are less approachable than the Gardner books. They are however full with interesting mathematical detective-like investigations ('when least is best' would be a good start).
Again, be warned the the Nahin books are packed with sometimes non trivial university level math.