If you have installed Python but forgot to check the Add Python to PATH option during the installation, you may still be able to access your installation with ease.
If you did not uncheck the option to install the py launcher, then you'll instead have a py command which can be used in the same way. If you want to be able to access your Python installation via the python command, then your best option is to re-install Python (remembering to tick the Add Python to PATH checkbox).
You can pass any options to the Python interpreter, e.g. to install the numpy module from PyPI you can run py -3 -m pip install numpy or python -m pip install numpy.
You can also access different versions of Python using the version flag of the py command, like so:
C:\Users\Username> py -3.7
... Python 3.7 starts ...
C:\Users\Username> py -3.6
... Python 3.6 starts ...
C:\Users\Username> py -2
... Python 2 (any version installed) starts ...