How do I know when to use Sine, Cosine and Tangent

The easy way to remember these uses the acronym SOH-CAH-TOA

The function to use depends on the sides of the triangle you know in the problem. 

With right angled triangles each side has a name: the hypotenuse is the longest side (the one that doesn't touch the right angle). The other sides are the opposite and the adjacent. Their names change depending on which side you are looking at. The opposite side is the one that doesn't touch your angle and the adjacent side does touch your angle.

The first part: SOH means if you are looking at the opposite side and the hypotenuse then you are working with Sine. Sin(angle)=opp/hyp

The second part: CAH means you use Cosine with the adjacent and the hypotenuse. Cos(angle)=adj/hyp

The last part: TOA means you use Tangent when you have the opposite and the adjacent. Tan(angle)=opp/adj