Learn Russian: Куда мы идём? У тебя есть план?

Both words mean 'where', but Куда implies direction or movement towards a place ('where to'). Because the sentence uses the verb идём (are going), we need the directional Куда. Где is used when you are already located somewhere ('where at').

'We are going' is a continuous action in English. Why is there no word for 'are' in мы идём?

Russian does not have a separate present continuous tense like English does. The single verb form идём translates to both 'we go' and 'we are going' depending on the context.

How does У тебя есть план? translate literally, and why is тебя used instead of ты?

In Russian, possession is expressed differently than in English. We use the preposition У (by/at) + the Genitive case of the pronoun (тебя) + есть (is/exists). Literally, it translates to 'By you there is a plan.'

Since the plan is the thing you possess, is план in the accusative (object) case here?

No, план is actually in the nominative (subject) case! Because the literal Russian structure is 'At you exists a plan', the plan is the subject that is doing the existing, not a direct object.