Well you're in the write place!
You an ask any questions you have about English grammar, vocabulary, usage, and etc here in the Questions and Answers of Italki.
You can practice your writing, like writing a story or essay, in the notebook sections.
You can find language partners and exchange languages with an English speaker that may be learning your native language Vietnamese :)
Finally, of course, you can purchase lessons from a community teacher or professional teacher.
Welcomee to Italki and good luck!