How should I know when to use "tu" or "usted"

Usted is a more formal and respectful term to use when talking to a new acquaintance, an older person or someone who you wish to show respect to due to their higher rank e.g an authority figure. It's plural is ustedes.Tu is an informal term which would be used when directing yourself to someone of the same rank, age or education. Also to someone younger than you. The plural would be vosotros.
Examples:¿Usted sabe dónde es la clase de matemáticas?¿Tú sabes dónde es la clase de matemáticas?They both translate as do you know where the maths class is but the question would be formulated depending on who the question is directed to.
As an extra bit of information and to avoid possible confusion, usted can be used differently depending on the origin of the speaker. The rules I explained above are how European Spanish would use the term but in South America usted is commonly used for tu as well, despite the situation being formal or informal.