In Spanish what letter do you drop to have a informal affirmative command?

For "normal" verbs, to form the informal plural command, you drop the "r" from the infinitive, and add a "d". Example: "hablar" becomes "hablad". If you are speaking if the plural informal (vosotros) form of a reflexive verb, the imperative is formed by dropping the "d" before addding "os". Example: "sentarse" yields "sentad", then "sentaos" (sit down).

For the imperative of the first person plural of a reflexive verb, the final "s" is dropped and the suffix "nos" is added. A written accent must be used to preserve the sound of the resulting word. Example: "irse", the reflexive form of "ir". "Vamos" is the First person plural. Drop the "s" and add "nos" and an accent, and the result is "vámonos". (let's leave, let's get out of here).