The longest side of a right-angled triangle is the hypotenuse
The other two sides are opposite or adjacent to a chosen angle.
Pythagoras's Theorem is that the side lengths always relate thus:
Ratios between sides are normally called sin, cos and tan.
The word soh-cah-toa can help you learn them!
soh means sin = o÷h
Knowing the lengths of sides, we can find a sin, cos or tan ratio. From this angles can be found using inverse-sin (or inverse-cos, or inverse-tan) on scientific calculators.