I'm looking for a shirt.
'Cerco' is pronounced 'CHER-ko' — the 'c' before 'e' is soft like 'ch' in English 'cheese'.
The simplest way to state what you need. Use immediately after greeting the assistant or in response to their offer of help.
'Cerco' (I'm looking for) comes from 'cercare', a regular -are verb. No article needed for the verb itself. 'Una camicia' — camicia is feminine, so 'una'.
Cerco un paio di pantaloni.
I'm looking for a pair of trousers.
Un paio di is always used with paired items
Cerco un vestito elegante.
I'm looking for an elegant dress.
Add adjective after the noun
Avete delle camicie?
Do you have any shirts?
More open question, lets staff show you options
Italian men's shirts are often described by collar style ('colletto italiano', 'colletto button-down'). Sales staff may ask about formality level — office, casual or smart-casual.