I’m sure this is very basic, but I am a newbie. Can someone tell me how to show a 45 deg miter?
It depends on what you are mitering. For example if you want to miter the end of a 2x4 across the wide direction you can select the 1-1/2 in. Long edge and move it along the length of the board 3-1/2 inches. For more complex shapes there are other options. If you share your model we can give you more specific methods.
To help achieve your goals, some time spent at the SketchUp Campus and at the SketchUp - YouTube channel will be very worthwhile. Both sites are from the SketchUp team. On the YouTube channel, pay attention to the Square One Series. It covers the basics for each tool.
Thank ylou for the advice Learned One, I will definitely do that.
Move tool as above:
Or draw a miter line and use Push/Pull.
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There are a million way to do a mitre, but basically it is half a square cut off the end.
A simple method without needing to measure or use guides is to Rotate Copy the top edge down to the side which will give you an endpoint at 45deg. Dave has given you two options and this is a third.
When you get to more complex shapes you just need to use a face at 45deg to cut through the geometry. No magic, just edges and faces. There are all sorts of tools and other ways, but these are the rudiments of the software.
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Follow you for more tutorials. 
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I’m thinking of a new series on how to replace tablet screens. ;^)
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This…
This gives me agida.
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The riving blade on my table saw is crooked so doing a 45 and Push-Pull there is too dangerous. Safety first!
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