How to install matplotlib?

[snippsat]

What should I do?

Follow this and install 3.7 Python 3.6/3.7 and pip installation under Windows
Make sure that Path point to 3.7,see Fixing Path if needed
Then it look like this from cmd.

Microsoft Windows [Version 10.0.17134.648]
(c) 2018 Microsoft Corporation. Med enerett.

C:\WINDOWS\system32>cd\

# Check that point to 3.7
C:\>python -V
Python 3.7.2

# Check pip point to 3.7
C:\>pip -V
pip 19.0.3 from c:\python37\lib\site-packages\pip (python 3.7)

C:\>pip uninstall numpy
Uninstalling numpy-1.16.2:
  Would remove:
    c:\python37\lib\site-packages\numpy-1.16.2.dist-info\*
    c:\python37\lib\site-packages\numpy\*
    c:\python37\scripts\f2py.exe
Proceed (y/n)? y
  Successfully uninstalled numpy-1.16.2

C:\>pip uninstall numpy
Skipping numpy as it is not installed.

# You do this,just showed how uninstall shall work
C:\>pip install matplotlib

So then I tried using python in visual studio 2017: Python 3.6.

Don't mess with visual studio 2017 use VS Code it's much more geared towards Python.
VS Code from start