@candle if you want to be formal or the person is really serious / somewhat "old" or of a "respectable age" you use vous. If you're flirting with a complete stranger use also vous. Either way use tu just like you would in any other cases

@candle « Tu » is for a close person, so you'll use it most of the time when you flirt with someone. You use « vous » when there is many person, or when you are speaking to a more distant person, or with more respect.