1. Read books
Read as much books as possible to learn the grammatical structure. The dialogues of the characters could be a good point of reference. Make sure that your Characters are not speaking the local dialects. Be careful in choosing the books as well. Make sure that the books are good and the writer is established. Reading newspapers has the advantage of getting two birds in one stone. Knowledge and language are the birds but the birds are caught alive.
2. Watch English movies and programs
Again the warning of dialect is applicable. Make sure that the actors are speaking good English. To avoid the mistake you shall watch English News channels.
3. Speak to the mirror.
Close the door. Stand in front of the mirror. Do the role-play imagining yourself as another person. Speak to the mirror. Perfect yourself with the knowledge gained by reading and watching. Understand the mistakes , make changes and perfect yourself.
4. Get the confidence boost
Tell yourself that you are the best. You should have the conviction yourself. Turn deaf ears to mockery.
5. Speak to your dear ones.
Your dear ones are the only ones whom you can rely on during your practice. Start speaking to them in English.
6. Think in English
Your thoughts, soliloquy and dreams should also be in English so that words come to you easily as and when needed. When thoughts start flowing, they pave the pathway for the words to come out of your mouth
7. Speak out
Start speaking in English with confidence. Adding you know, like, what do you say, i mean etc are not so bad. When you gets stuck, using these small small words are far better than the humming of uuh, uuh. Try to reduce the frequency with time.
These are all just the simplest way to speak the colloquial language. If you really want to learn the authentic American or British accent, better got for IELTS or TOEFL