Dear Tran Tin Nghi, I suggest you have a look at the following concepts and decide to which one/s you feel interested. Then, you´ll have started the very first step: Dialect, Figurative language, Foreign languages, Form classes, Homographs, Neologisms,
Nonstandar English (or any other language), Numbers (numerals), Phrases, Sentences,
Sign language, Vocabulary, Written language, Bilingualism, Grammar, Language Develpment, Literacy, Monolingualism, Multilingualism, Symbolism, Verbal communication, Information Retrieval, Natural Language, Controlled Vocabualry. Cheers!