We use “Señor” and “señora” plus the surname. For example, el señor López. We use “don” and “ doña” plus the first name. For example, doña Carmen.

Doña and Don are usually used when referring to older people. "Don Juan", "Doña María". Let's say it's a polite way to call and show them some additional respect. Hope it helps. Pablo

I would add that when you meet someone that requires Don or Doña as an honorific, they will probably be introduced with that as part of their name. This makes your job easier.

Don and Doña, in my country is used in a friendly way when you know the person but you arent to close, usually to older people than you to show respect ( an older neighbor, your new girlfriend parents, your friends parents) Take note that you must use with the first name in contrast with Mr. that you use with the last name.

Don es para referirnos a un señor, un hombre, mientras que Doña, es para referirnos a una señora, una mujer. Por ejemplo, Doña Maria vende vestidos muy bonitos y Don Jose vende trajes de graduación.