I am a complete novice with QGIS.
I have completed my first project in QGIS 2.18.9 on a Mac. I need to transfer it to another Windows computer.
How can I best do this while retaining all my settings and plugins etc.?
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Do you need to keep settings of the project, or settings of QGIS itself?
To save your project with all its layers, groups and styles I recommend using QConsolidate plugin - it will copy all of your files including .qgs itself to a single folder (don't forget to check "Show also experimental plugins" checkbox to see it).
As for moving plugins - please follow @Joseph 's recommendations above and this answer.