Ok how do i talk to a girl without being nervous?

ok when me and a girl are alone i can kinda talk to her but sometimes the conversation go's died and when they friends come around i get quiet because i dont know what to say they sometimes be talking about stuff they did or stuff i can comment on like some person with a funny hat sometimes i laugh at it in it make me look stupid because i be quiet for like 20 min and bust out laughing at something funny they said.. so how do i get comfortable around them and i cant always and also what kind of stuff sould i be talking about