Here's a good rule of thumb for professional contexts:
Use “Usted”, then observe carefully to see if/when “tú” is appropriate.
Most Spanish students understand the difference between the informal "tú" and the formal "Usted." And most students can judge situations in which informal address is appropriate (peers, children, etc.) and when it's better to use the formal "Usted" (older people, those in position of authority, etc.).
However, just choosing between the two terms "tú" and "Usted" is only the tip of the iceberg. You also have to manage the corresponding verb conjugations, possessive adjectives and direct/indirect object pronouns.
For example, here's how some different verbs look:
te llamas se llama
tuyo/a –s suyo/a –s
te lo / la
Here is an example of what it might look like to introduce yourself to someone and start a basic conversation using Ud.:
“Lo siento. No sé su nombre. ¿Cómo se llama Ud.? Es un placer conocerlo/la. ¿De dónde es? ¿Le gusta Carolina del Norte/Chapel Hill?.”
As you can see, it's a lot more than just choosing between the two terms "tú" and "Ud."