GCSE Trigonometry: choosing the right method
Trigonometry becomes much easier when you first identify the triangle, mark what you know and choose the relationship that matches the information given.
SOHCAHTOA for right-angled triangles
For a right-angled triangle, identify the hypotenuse first, then label the opposite and adjacent sides relative to the angle you are using. Choose sine, cosine or tangent from the two sides involved.
Sine rule or cosine rule?
For non-right-angled triangles, the information given determines the method. Matching a side with its opposite angle points towards the sine rule; two sides with the included angle, or all three sides, points towards the cosine rule.
3D trigonometry
In a 3D problem, do not try to calculate from the whole solid at once. Identify the relevant plane, draw or trace the right-angled triangle inside it, then use Pythagoras or trigonometry on that 2D triangle.