Don't worry too much about how you sound! People throughout the United States have varying accents. I suppose there are standards in the media, though.
I suggest enunciating your words and paying close attention to the phonetic system. You're probably having a difficult time with the inflections (rising and falling of the voice). What is your native language? As long as your language isn't tonal, you shouldn't have too much trouble resembling an American or British speaker.