How do i start a conversation?

so my social skills suck!! when i say i don't know how to start a conversation, i literally don't know how. i only speak if spoken to, whether its my family or friends (i only have 3 friends). i keep to myself alot because i don't feel like an interesting person to talk to, and im not funny. so how do i start an INTERESTING, FUNNY, ENJOYABLE conversation ? and not a grown up conversation but more of a teen conversation. im 15 and idk what teens talk about. some people invited me to a kickback and im extremely nervous because i don't know any of them like that. the only reason they invited me is because i smoke weed, and they think im a chill person from what they judged on my outerlook. so what are topics i can bring up ? how do i keep the conversation going? and how do i leave a good impression on them not a boring one.