this is very normal for a new english speaker. i had the same problem, when i first came in abroad. usually, i didnt even understand what they are saying. most interesting part, the american people, never uses full verb, they mostly uses slang. beside black people, they talk in their traditional way, which is sometimes very hard to understand. i dont know where you are now? or do u want to go abroad to study?
my first suggession for u will be, never copy them..whatever they speak, u just try to understand. and then u make it clear urself. when you dont understand, tell them, its ur second language and speak slowly. they will help u out.
beside when u speak english, make a eye contact with them. and show them ur confident. if u dont do it, ur mind will think something else, and u will lose the track. specially, try to organize the sentence in ur language in ur head and then translate it in english. it just a matter of time. u will find that ur using the same thing again and again. then after a time period u will know, what to say or how to speak.
give youself time. listen to t.v or radio, when u do nothing.
or practice with ur friends.
i hope it will help u out ....good luck..