When you're talking to a girl, don't try to change the subject too quickly. Ask her something then say or ask something depending on what she says. Don't ask random questions that don't relate much to what she said. Just feed off what shes says. Also, try not to think about the "right" thing to say because this will probably end up in you getting nervous and looking stupid. Have CONFIDENCE IN WHAT YOU SAY. Even if it sounds "stupid" because that may just be your mind trying to psyche you out. If you're a good guy, it will show eventually so don't rush through things by trying to be funny or asking random questions