How Do You Say You In Icelandic?

Singular: Þú (pronounced "thuu", soft th sound as in think)
Plural: Þið (pronounced "thith", first th is soft, second is hard as in the)

You might also find þig, þér and þín in the singular (accusative, dative, genetive case) and ykkur and ykkar in the plural (accusative and dative, genetive case).