I need help with the map.
'Aiuto' — a-YU-to. Three syllables; the 'iu' is a diphthong not two separate vowels.
Use this simple statement to approach a station attendant or fellow traveller when you are confused by the metro map. It is softer than asking a direct question and invites the other person to help you.
'Mi serve' is an impersonal construction using 'servire' meaning I need or I require. It works like 'mi piace' — the thing needed is the subject. 'Ho bisogno di aiuto' is a more direct alternative. Both are correct; 'mi serve' sounds slightly more natural in spoken Italian.
Non capisco la mappa.
I don't understand the map.
States the problem without explicitly asking for help.
Può aiutarmi con la mappa?
Can you help me with the map?
Direct polite request using the Lei form.
Come funziona questa mappa?
How does this map work?
A less direct way of asking for an explanation.
Piazza Navona has no metro stop directly underneath it. The nearest stops are Spagna (line A) or Barberini. Romans are generally happy to help tourists with directions, especially if you approach politely.