"No I will not, I do not want to go."
Translation:Na fydda, dw i ddim eisiau mynd.
No. rydw i... is only used in positive statements. dydw i... is only used in negative statements. dw i... can be used in either.
isio, isie, isia, eisia and so on are just various common pronunciations of the word eisiau. They are so common that they often get used in informal writing as well as in conversation. eisiau is rarely pronounced anywhere in Wales as it is written!