Shop assistants address customers with polite Lei — and it's polite to answer the same way. You'll hear:

The assistant	English
Buongiorno, posso aiutarla?	Good morning, can I help you?
Desidera? / Mi dica.	What would you like?
Che taglia porta?	What size are you?

And you can say:

You	English
Sto solo guardando, grazie.	I'm just looking, thanks.
Cerco un vestito per una festa.	I'm looking for a dress for a party.
Quanto costa? / Quanto costano?	How much is it? / are they?
Mi scusi, …	Excuse me, … (Lei form)

Remember the verb agrees with what you're asking about: quanto costa la giacca, but quanto costano le scarpe.