Spend a lot of time with native speakers of English. Observe and make a note of the way they use words and expressions. Imitate their way of speaking and tone in your conversation. Practice speaking like this with your family.
Watch English movies, news and talk shows on television. Pay attention to common expressions that are a part of informal English. For instance, people will say "There you go" while giving something. When you hear someone ask "What do you mean?" it means they are asking for an explanation. If someone says "Come again?", they are asking you to repeat something, not to visit again. Learn these and the other common expressions you hear and practice using them in your conversations with friends to make them a natural part of your lingo.
Get active on social networking sites, and read blogs and online forums regularly to study how people express their thoughts. Observing the way people conduct online conversations is an excellent way to learn informal English.
Master a few basic guidelines to be able to speak informal English. Using entire auxiliary verbs is formal; using a contracted version of the same verb is informal. So it is formal English to say, "I have worked in Washington for many years", and informal to say, "I've worked in Washington for many years." Another feature of informal English is to speak without relative structures. For instance, the formal sentence "She sensed that he was uncomfortable" becomes informal when you say "She sensed he was uncomfortable." Avoid using passive structures; use the active voice to sound more informal.
Don't hesitate to bend the rules of grammar. Omitting the auxiliary verb totally and substituting the word 'who' for 'whom' are common examples of language that is incorrect but accepted in everyday usage. Practice saying things in this manner to sound more informal. Instead of asking, "Have you finished studying for your exams?" ask, "Finished studying?"