You can however say "ce soir, je m'habille EN pantalon" (tonight, I'll get dressed in pants) but the focus here is that you are wearing pants and not a skirt, so you won't talk about the precise pair of pants you are going to be wearing (you cannot say "ce soir, j e m'habille en pantalon noir et court"... for this you need to use "porter" ou "mettre").
For this notion, we also use the expression “être en”.
Example: ce soir, je suis en pantalon. Je m’habille en pantalon. (Tonight, I’m wearing pants).
An idiomatique use of the verb s’habiller means “to dress up”.
Example: est-ce que je dois m’habiller pour aller chez Anne ? (should I dress up to go to Ann’s)
2 - Porter and Mettre
To describe what you are wearing, use the verbs porter (to wear) or mettre (to put on).
Example: Ce soir, je porte un pantalon noir et un pull rose.(Tonight, I’ll wear a black pair of pants and a pink sweater)
Example: Demain, je vais mettre une robe bleue. (Tomorrow, I’ll wear a blue dress)
3 - Other related verbs:
Other French verbs commonly used with clothes are : enlever (to take off), retirer (to take off), enfiler (to slip in), essayer (to try on), se changer (to change clothes), rester en... pyjama (to stay in... my pyjamas)
4 – Common French expressions
Note these common expressions that can be a bit confusing:
“Comment tu t’habilles ce soir?” what are you going to wear tonight?
“Qu’est-ce que tu mets demain?” what are you going to wear tomorrow?
“Elle portait quoi ?” what was she wearing?
“Je ne me change pas, je reste en jean” I won’t change outfit, I’ll keep wearing my jeans.
5 - French Idioms:
"Il est sur son trente-et-un": he is dressed very chic
"Je n’ai rien à me mettre": I have nothing to wear.
I suggest you check out my free French audio books, particularly my beginner audio novel "À Moi Paris", chapter 4 - Sophie Cherche une tenue, which talks about shopping.
You might also enjoy my post on How to dress in Paris.
