"Beviamo tè al limone."
Think of it like this: the preposition (to) is a but if you add an article (the) it becomes alla, al, allo, ai.... The preposition + article combo matches the noun's gender and number.
- Vado al ristorante/I go to the restaurant
- Andiamo allo zoo/We go to the zoo
- Scrivo alla donna/I write to the woman
- Mario legge all'uomo/Mario reads to the man
I have always seen this as "beviamo te con limone," not "al." I'm also using the Fluenz program (highly recommended) and this is what they teach.