Have you seen the ratios SOH CAH TOA?
So that you're on the same page, you need to have your reference point be the smallest, or "most acute" angle in the triangle.. The mnemonic stands for [Sin=Opposite/Hypotenuse] or "SOH", CAH is Cos=adjecent/hypotenuse, TOA is Tangent=opposite/adjecent
In a right triangle, the (hypotenuse) is the side of the triangle that is directly opposite of the right angle. The (opposite) side is the side of the triangle furthest away from our reference angle. And adjecent means next to, so it's the side that (is the only side left and) is next to our reference angle which is NOT the hypotenuse.
Hope that helped.