yes, it means I like you in a teenage boy/ girl romantic way. Don't say to your teacher, or your boss " I fancy you" if you like them, unless you mean you have romantic feelings for them... ( well maybe you do fancy them...but keep it to yourself!)

Just "I fancy you" means that I have taken a liking to you or just that I like you. If the phrase has a clause after it, it means that you imagine something or can form a picture of it in your mind. "I fancy you'll want to get to bed after such a long day." = "I imagine you''ll want to get to bed.." or "I'll bet you want to get to bed..."

It means "I like you."