How do I put in exact dimensions to my parts?

Just loaded sketch up. Drew a 3D rectangular box. Want to hollow it out and design it to size. Only seem to be able to draw a solid huge block. Does not seem a method to edit anything or put in dimensions. It is taking me ages to find simple edit controls. Are they hidden? The thing I am designing is a U shaped section from the end view with 1 inch faces on the front view.

You should probably stop trying to draw and learn how to use the tools. There are some good basic videos available under Learn on The controls are not hidden.

Injecting Accuracy Into Your Model — Aidan Chopra

Accuracy — SketchUp Video