* If you're unsure of marital status and want to be polite: Use κυρία (kyria). This is the equivalent of "Madam" or "Mrs." and is a respectful way to address a woman regardless of her marital status.
* If you know she's unmarried and want to use a term that suggests youth (though less common now): You could theoretically use δεσποινίς (despoinís), but it sounds quite formal and is rarely used in modern Greek. It's an older, more formal term and can feel stiff or even outdated.
In almost all cases, κυρία (kyria) is the safest and most appropriate way to address a woman in Greek. You wouldn't typically use a direct equivalent of "Miss" like you would in English.