"You eat a sandwich."
Translation:Tu mangi un panino.
Nope, as mukkapazza pointed out 'voi' should work, just make sure you conjugate the verb properly, that it 'voi mangiate un panino'. Mind you 'you eat a sandwich' can also be translated as 'Lei mangia un panino' in which case the pronoun should always be capitalised, no matter where it falls in the sentence just like I in English, and it would be the formal you.
You can't mix and match the subject and verb like that.
Formality or informality is not the question here as both Tu and tu are informal. The only reason the first example is capitalized is because if starts a sentence. Not to be confused with the formal Lei or the informal lei - and that can be confusing when one of them starts a sentence! Understand also the verb conjugation: Mangi = you (informal singular) eat. Mangia = He/She/It eats. Hope this helps.