Learn Spanish (Spain): Me lavo el pelo en el baño.

Lavarse is a reflexive verb, meaning the action reflects back onto the person doing it. Because you are washing a part of your own body, you need the reflexive pronoun me to show who is receiving the action. If you were washing your car or your dog, you would just say lavo without the me.

Why does it say el pelo (the hair) instead of mi pelo (my hair)?

In Spanish, when you use a reflexive verb to talk about daily routines involving your own body parts or clothing, you use a definite article like el or la (the) instead of a possessive word like mi (my). The me in me lavo already makes it perfectly clear whose hair is being washed!

Can I put the me after the verb, like lavome?

No, not in this sentence. When a verb is conjugated (changed to match the subject, like lavo for 'I wash'), the reflexive pronoun me must go as a separate word directly before it. You can only attach reflexive pronouns to the very end of a verb if the verb is still in its full dictionary form, like lavarse.

Does el baño mean the bathroom room, the bathtub, or the toilet?

It can mean all of them depending on the context! In this sentence, el baño refers to the room itself (the bathroom). In Spain, it is also the most common word you will use when asking where a public restroom or toilet is.