A yes/no question is answered by repeating the verb, not with sim/não. A bare 'sim' sounds curt or foreign.
- Tens fome? — Tenho.
- Are you hungry? — I have.
- Já almoçaste? — Já.
- Have you had lunch? — Already.
- Queres vir? — Quero.
- Do you want to come? — I want to.
What to actually say at the counter. 'Um café' is an espresso; the milky ones each have their own name.
- uma bica
- an espresso — the Lisbon word for it
- um galão
- milky coffee, in a tall glass
- uma meia de leite
- half coffee half milk, in a cup
- um abatanado
- a long black
'Cerveja' is correct everywhere; ordering the small draught by its local name is what marks you as not a tourist.
- uma imperial
- a small draught beer
- um fino
- the same glass, said in the north
- uma caneca
- the big one, about half a litre