Trig ratios can only be used on right triangles with acute measures.
If given an angle and there are adjacent and opposite sides, then use tan(opposite/adjacent)
If given an angle and there is an adjacent side and a hypotenuse, then use cosine(adjacent/hypotenuse)
If given an angle and there is an opposite and adjacent side, then use sin(opposite/hypotenuse)
A common mnemonic device used to memorize the trig rules is SOH-CAH-TOA
