Crush/Flirting

So I’m a freshman in high school and there’s this boy that I just met. He used to go to the same middle school with a lot of my friends, so it’s not that awkward to talk to him, but he used to have a crush on my bff....😭. He’s weird because one day he’ll be literally the nicest and sweetest person ever, but then I’ll talk to him the next day and it’s like I’m talking to a different person. I personally want to make the first move and start the flirting or something... but I just don’t know how to do subtlety. PLEASEEEEE HELP ALL YOU CAN!!!! YA GIRL NEEDS IT!!!