Hello Rustum, if you are asking about children who learn English as their native language, then it's safe to say we learn English the same way all human children learn their native language. The first couple years are spent listening to our families and making sounds (or baby talk), then we start by saying a word at a time, gradually increasing the number of different words we can say. Then we string together simple sentences until we can say more and more complicated sentences. By the time we're around four years old, we can speak simple English with our families, teachers, classmates.
Although we do learn grammar rules in primary school, most native speakers gain an intuitive grasp of grammar that comes from daily practice for years. (We know what sounds right even though most us could not explain why or be able to name the grammar principle we are using.) Vocabulary, idioms, figures of speech and more complex language patterns are learned primarily from whatever media and people we are exposed to most as we grow up.