Practice makes perfect – Practice often inside and outside of the classroom. Speak English every day. Listen, read and write in English as much as you can. Create a schedule for yourself to practice.
Learn to think in English – When you look at something try to think of the English word for it. Try to make simple sentences in your head and have simple conversations with yourself (in your head). This video (with subtitles in Vietnamese) has more ways to think in English.
Listen in English – For at least 20 minutes every day (audio book, podcast, songs, radio, etc). More ways to practice listening here.
Very important – Don’t be afraid to make mistakes, learn from your mistakes and ask a lot of questions.
Read! – English books, articles online, etc. Harry Potter is a good book to start with. More reading suggestions here.
Start an English diary – Carry a small notebook with you all the time and write new words in it (with the translations in your native language). To remember the new words, cover the English word with your hand. Then, look at the word in your native language and see if you can remember the word in English. Use a pen and paper in combination with your smartphone (not only your smartphone). When you learn a new word, you should also learn how to make a sentence with it.
Write in English – Write short paragraphs. Ask a native English speaker to correct it for you. Either online or in-person.
Don’t forget to have fun! – Play English games (apps or with your computer). Here are some games to play online.
Here are a lot of links for students to study English at home.