Ustedes for everyone, and he comido or comí

Ustedes for everyone

Latin America has no vosotros. Ustedes is the plural you for everyone, friends and strangers alike, always with the third-person plural verb:

The Canary Islands and parts of western Andalusia say ustedes to friends too. Popular speech in western Andalusia even pairs it with the vosotros verb (ustedes tenéis); the standard everywhere is ustedes tienen. If you learn Latin American Spanish you can leave vosotros out of your own speech, but you still need to recognise it: Spanish films, books and songs are full of it.

• ¿Qué queréis comer? - What do you (all) want to eat? (Spain, friends)
• ¿Qué quieren comer? - What do you (all) want to eat? (Latin America)

Today's past: he comido or comí?

For something that happened today, most of Spain uses the perfect. Most of Latin America, the Canary Islands and north-west Spain use the preterite:

Both are correct Spanish. Latin America still uses the perfect for a whole life up to now: Nunca he estado en España. - I have never been to Spain. That sentence works on both sides of the Atlantic, though Argentina also says nunca estuve. The full rules are on the present perfect.