Use of Senorita

Just say Buenos días, Buena tarde, buenas noches. No need for the señor, señora, señorita. You’ll be understood, especially if you have no idea whether the female is married or single.

Senor/Senora are deemed too formal for most, and indicate an older person typically, so not well taken if they are younger. As the previous post indicated, a simple greeting suffices.

The other thing commented on about visitors, is over using Por Favor (please) and Gracias (Thank You), once in an interaction is more than sufficient, and if you listen to Spanish speakers asking or receiving something the please/thank you is inferred in their tone, rather than said.

Just say "Buenas" or "Hola" or "Hola, buenas."

That's much more common, you don't need to think about what time it is, and what people would be prefer to be called. You want to communicate with people in informal Spanish in most situations.

I don't think we ever use "señorita", maybe occasionally in a department store?

I think these days it can sound a bit condescending too. Would just stick with buenas or hola if you're greeting a shopkeeper or something like that.

I agree with Carlos.

Furthermore, suppose you're visiting one of the areas that has its own language (Galicia, the Basque Country, the Balearic Islands, Valencia, or Catalonia). In that case, locals will warm up to you if you say it in their language instead. That's certainly the case in my turf, in Catalonia.

So, when in Barcelona or anywhere else in Catalonia, say "Bon dia" (bon-dEEah) or "Hola" when greeting someone --don't worry about the time of the day, we understand you're making the effort--, "Si us plau" (see-oos-plAoo) for 'please', then "Mercès" (mer-sEs) or "Merci" (mEr-see) when thanking [1] and finally "Adeu" (ah-dEh-oo) or simply "'deu" for 'good bye' / 'bye'. Easy peasy!

Always greet when entering and leaving a place.

These are the very basics of Catalan etiquette. ;)

[1] Note our "merci" has nothing to do with French, it's the abbreviation of "mercès" and it's also pronounced differently than in French. The form "gràcies" (grAh-see-s, with an '-es' sound at the end, not '-as' like in Spanish) also exists in Catalan, and can be used, but "merci" is more common, especially in the area of Barcelona.