Well, most children don´t learn grammar as such and they speak perfectly well. It mainly comes from practise. As a Portuguese learner I can say that using the language is more important than learning the specific rules of grammar. Many grammarians argue amongst themselves over the rules of English usage, so my advice would be to expose yourself to language resources - such as radio, music, film and books, read a lot and whenever you have the chance - practise conversation but do not be afraid of making mistakes - we all make those, even native speakers use incorrect sentence constructions!