I need a girls advice, i'm a guy in grade 9?

as it says i'm a guy in grade 9 15 years old and i suck at going to talk to girls. normally there are groups of chicks not single only plural. its a problem because i normally don't know what to talk about. the only thing i want to know is topics i can talk about when i first talk to them, like its a first impression pretty much. i don't know what to talk about and these chicks are always smiling at me or calling me to come over but its a challenge and i know girls are humans and i can talk about anything but its not that easy so anyways does anyone know any topics i can talk about or some funny jokes to crack a few