Angles in standard position
An angle in standard position starts at the positive x-axis; its quadrant fixes the sign and its reference angle fixes the size.
A worked example
An angle of 225° lies in quadrant III, where the reference angle is 225 - 180 = 45°, so sin 225° = -0.707.
The mistake people actually make
Sine is negative in quadrants III and IV, cosine in II and III, so a bare calculator value can carry the wrong sign. In an angle-of-elevation question measured from eye level, add the observer's eye height to get the full height.