Soh Cah Toa HELP PLEASE!!!!or ill faill my math test!!!?

so i get soh cah toa and how to figure it out, but one thing i dont understand is which side u need to use for the sin cos or tan?which sides do u choose? the one that is not given? and then which other side if im given to sides and one angle and the hypotenuse is missing????i dont understand plz help!!!