Learn French: Lave-toi dans un bain chaud avec une bougie. - Wash yourself in a hot bath with a candle.

When giving an affirmative command (telling someone to do something), French moves the reflexive pronoun to the end of the verb and attaches it with a hyphen. When this happens, te changes to the stressed pronoun toi.

Why is there no 's' at the end of Lave if it is the tu form?

For regular -er verbs like se laver, the final 's' is always dropped in the tu form of the imperative. So, the normal statement Tu te laves becomes the command Lave-toi.

Could I use Lavez-vous here instead?

Yes, if the context changes! Lave-toi is informal and used when talking to one person you know well. If you were speaking formally to someone, or talking to multiple people, you would use the vous command form: Lavez-vous.

Could I say dans une baignoire instead of dans un bain?

You could, but it slightly changes the feel. La baignoire refers to the physical bathtub itself, like the piece of furniture. Un bain refers to the bathwater or the relaxing experience of taking a bath, which fits the mood of this sentence perfectly.