Most 3D questions are really about a hidden 2D right-angled triangle inside a cuboid, prism or pyramid. First decide which flat triangle you can draw and label.

Use Pythagoras when you need a missing side: $a^2+b^2=c^2$. Use SOHCAHTOA for angles, and choose the ratio from the sides you actually know.

A space diagonal goes through the inside of a solid, while a plane is just a flat face. The angle between a line and a plane is measured using the line's projection, like its shadow, on that face.