"I have a dress."
Translation:Io ho un vestito.
Uno is used when referring to a singular masculine nouns with definitive article "lo", i.e. lo stato (the state), uno stato (the state). It would be useful to look up nouns with impure consonants, that is, s+consonant (impure s), gn, pn, ps, x or z; and pay attention that this rule only applies to singular masculine nouns. :)