In Spanish you use the pronoun usted to show respect when talking to the elderly or a person with a title such as a professor, a doctor, etc. On the other hand, you use the pronoun tú when talking to a peer or younger person.
A phrase to let know a professor he is next in line you could say: Profesor Solís, usted es el siguiente. (Professor Solís, your up next. )
To tell a friend of yours that he is up next, an appropriate phrase could be: Juan, tú eres el siguiente. (Juan, you are up next.)
For example, if you want to let your aunt know that she is in charge of deciding about buying pizza: Tía usted es la encargada de decidir. (Aunt, you are in charge of deciding about that.) To tell something similar to a child you can say : Luis tú tienes que decidir acerca de eso (Luis you have to decide about that)