The primary trigonometric ratios
SOH CAH TOA: sine is opposite over hypotenuse, cosine adjacent over hypotenuse, tangent opposite over adjacent.
A worked example
A ramp rises 2.4 m over a run of 6.0 m. tan θ = 2.4 / 6.0 = 0.40, so θ = 21.8°.
The mistake people actually make
Label the sides relative to the angle you are working with. The opposite side changes when the angle does.