In order to express your misunderstanding, just say:
¿Perdón? – Sorry?
¿Cómo dice? – Excuse me? Sorry?
No entiendo – I don´t understand
Spanish speak very fast. You can ask them to speak more slowly:
Perdone, ¿puede hablar más despacio? – Could you please speak more slowly?
¿Puede repetirlo, por favor? – Can you repeat it, please?