Formal:
* Je suis spécialisé(e) en... (I specialize in...) - This is a very formal way to say it and emphasizes your expertise in the field.
* Je suis major en... (I am majoring in...) - This is a more direct translation of "I am majoring in" and is also quite formal.
Informal:
* Je fais un major en... (I am doing a major in...) - This is a more colloquial way to say it and is more common in informal settings.
* Je suis en... (I am in...) - This is the most informal way to say it and is only appropriate in very casual conversations.
Example:
* Je suis spécialisé(e) en économie. (I specialize in economics.)
* Je suis major en littérature. (I am majoring in literature.)
* Je fais un major en informatique. (I am doing a major in computer science.)
* Je suis en sciences. (I am in sciences.)
Note: The gender of the adjective "spécialisé" will change depending on whether you are male or female.
You can also use the verb "étudier" (to study) to say "I am majoring in" in French.
Example:
* J'étudie la médecine. (I am studying medicine.)
* J'étudie les arts. (I am studying the arts.)
Ultimately, the best way to say "I am majoring in" in French depends on the situation and your personal preference.