Not a silly question.
I assume you are talking about right angled triangles?
If so, you must firstly and most importantly know how to label the sides of the triangle in relation to the angle in question. If that is not a problem consider the trigonometric ratios.
To decide which raiot to use inspect your triangle. If it has information on sides O and H then employ sine. When I say information, this could be a value or an unknown.