Here's a breakdown:
* Sei: You are
* totalmente: totally
* meraviglioso: wonderful
* mi piaci: I like you
* molto: very much
You can also say:
* Sei fantastico, mi piaci molto. (You are fantastic, I like you very much.)
* Sei bellissimo/a, mi piaci molto. (You are beautiful, I like you very much.)
The choice of wording will depend on the context and the relationship between the speakers.