I don't think you technically need to write "sinä"!
Nah, you don't. But you can. But you don't have to :p
No you don't...
Why isn't "No, oletteko valmis?" not correct?
Hmm, technically the word "valmis" should also change there, that is, the sentence becomes "No, oletteko (te) valmiita?".
This (oletteko valmiita?) is in case you are talking about plural you. In singular you, if you are being polite and use "teitittely", you should be able to say "oletteko valmis?".
"Oletteko valmis?" would be correct if you were addressing one person in a formal, polite register (plural verb but otherwise singular forms). It is fairly rare nowadays, though, and I'm not sure if the "no" in the beginning goes well with the tone.
Why i can't use " onko sinulla valmis?"
Because Onko sinulla…? means "Have you (got)…?" which pretty much requires that the object should be a noun, but valmis is an adjective.