If you installed Python with the default options, you will probably need to add Python to the PATH variable. This let's your operating system know where to look for the Python executable when you try running it.
To add Python to your PATH variable:
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Find the path of python.exe on your system. Look in these directories or search for it.
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Open System Properties and click on the Advanced tab
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Click on Environment Variables
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Under System variables find and click on the Path variable
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In edit mode, go to the end of the line and add ;C:\Python37 or whatever folder python.exe is in. Note the semicolon before the path; this will separate it from the previous path.
Follow the instructions here to check that the PATH variable was set properly.
If you are having problems:
Search the internet for "Add python to path on Windows 10 / Vista / XP / etc" to find instructions for your version of Windows.