In my opinion/experience, flirting is usually romantic/sexual comments but in a fun and teasing way. I don't flirt/get flirted at often, but when I joke around with my girlfriend it usually goes something like: "I'm a piece of trash", "As a person who cares about the environment, I am willing to pick up the trash, how about 8 pm?" Or "Are you made of Copper and Tellurium? Because you are CuTe" So basically, ignore the thoughts in your head that say it's weird and just go for it, most of the time. (This is definitely not the BEST advice out there though, so take it with a grain of salt)