Here "watch" is an idiom that mean "pay attention to." Alternate translation: "I will pay attention to the things that I say" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here "tongue" refers to the writer's speech. Alternate translation: "so that I do not speak an offense against Yahweh" (See: [[rc://en/ta/man/translate/figs-metonymy]])
To "muzzle" means to keep a mouth shut. Here David means that he will not speak while he is with an evil person. (See: [[rc://en/ta/man/translate/figs-metaphor]])