Tom,
Like you I have never done any 3D drawing work before. The only experience I have of even 2D drawing goes back an 'O' level in Technical Drawing 30 odd years ago!
However, I do find SketchUp an easy tool to master, and find that the ability to enter the size of an item manually really helps.
For example if I want a cuboid of 0.2mm x 0.6mm x 0.15mm, I would choose the rectangle tool to draw a rectangle of any size, then type "0.2mm,0.6mm<RETURN>", this can then be push/pulled to the depth of 0.15mm by pulling to say 0.3mm, then typing "0.15mm<RETURN>" which will reset the manually pulled depth to the entered figure (SketchUp doesn't unfortunately allow (unless someone knows different) setting the "snap" to a finer resolution than 0.1mm).
Ian