Before starting to learn NLP, prerequisites are basic knowledge of linear algebra, statistics, and python. You can enroll in free online tutorials available before starting for not facing any problem understanding NLP concepts.
Enroll a course online for NLP and learn the theoretical concepts. Try to read and understand the packages and libraries like NLTK, SpaCy, Stanford CoreNLP from their official websites. Then try to do a project using the concepts you learned. Write blogs explaining your project in detail. Also, add your projects to GitHub, it will add a good value to your profile.
In case, if you are interested you can enroll in this end-to-end certification program on NLP training in python.
You can watch this video to know more about NLP in python for a kickstart:
You can also learn about applications of the subfield of linguistics on applications of NLP.