First thing refers to the initial action or task completed at the start of a day, event, or process. There are several synonyms for the term that can be used depending on the context and situation. Some of these alternatives include "priorities," "top priority," "initial step," "primary task," and "key assignment." Each of these phrases can be used in place of "first thing" to convey the same idea of a crucial or essential action that must be taken promptly. Choosing the most appropriate synonym depends on the specific context and intended meaning of the statement.