Pip stands for pip installs package or pip installs Python.
You can see how to install pip and nltk at this link:
https://github.com/jalajthanaki/NLPython/blob/master/Appendix3/Installationdocs/App3_1_Install_python_pip_NLTK.md.
Python has been preinstalled on Linux and macOS. I would recommend you to use Linux OS or macOS as it will be easy to set up any ML-related frameworks.