What is the main concept of using lexical,linguistic, semantic or syntactic approach in NLP for cyberbullying
I suggest to check out some introductory books on NLP, e.g. Natural Language Processing with Python. This is a very accessible and practical introduction and useful even if you won't be working in Python. Another, more detailed text book is Speech and Language Processing by Jurafsky and Martin.

You need to understand the basics first, and these are independent of your particular application (cyber bullying). And I think this is too much knowledge to reasonably fit into a response here. But just to give you an idea:

Lexical: information about the words (meaning and structure)

Syntactic: the structure of the text (how words make up phrases, and how phrases make up sentences, the rules that govern that process)

Semantic: about the meaning of text (as it is constructed from the meaning of the words and their composition).

These are usually part of a pipeline, which means you need start processing from the words, then look how they form sentences, and then you can look at the meaning.

Your understanding of POS-tagging seems off the mark: it is the process of assigning words in a text their part of speech (POS), e.g. verb, noun, adjective etc. As such it is part of the syntactical processing (but requires lexical knowledge too), but is also useful for semantic analysis further down.