Learn Icelandic: Ferð þú til Spánar á morgun? - Are you going to Spain tomorrow?

This is because of the preposition til (to). When you use til to talk about going to a destination, it forces the noun that follows into the genitive case. The dictionary form of Spain is Spánn, but the genitive form is Spánar.

I learned the word for 'go' is fara. Why is it ferð here?

Fara is the basic dictionary form (to go). Icelandic verbs change their shape based on who is doing the action. Since the subject is þú (you), the verb changes to match it: þú ferð.

Where is the word for 'are' in 'Are you going'?

Icelandic doesn't use a separate 'are doing' or 'am doing' structure like English. Ferð þú covers both 'Do you go?' and 'Are you going?'. You simply put the action verb first to make it a question.

How does á morgun mean 'tomorrow'?

Literally, the two words á morgun translate to 'on morning'. However, this exact phrase is the standard way to say 'tomorrow' in Icelandic.