你可以使用以下几种方式来表达你的专业：

My major is...

例如：My major is Computer Science.

I major in...

例如：I major in Mechanical Engineering.

I'm majoring in...

例如：I'm majoring in accounting.

I was a... major.

例如：I was a biology major.

根据你的专业选择，可以选择以上相应的方式来表达。例如，如果你学的是计算机科学，可以说：“My major is Computer Science.” 或者 “I major in Computer Science.”