In Spanish, you say “sentarse” when you want to express the action of sitting down. It’s a reflexive verb, which means it often includes the pronoun that corresponds to the subject of the action.
If you want to instruct someone to sit down in a more direct way, you can say “¡Siéntate!” when addressing someone informally, or “¡Siéntese!” when addressing someone formally or speaking to a group. The formality and respect conveyed by the latter can be especially important in certain contexts, such as in professional settings or when speaking to elders.