Formal:
* I have studied English.
* I have been studying English. (implies ongoing study)
* I am a student of English.
Informal:
* I studied English. (implies past tense, not necessarily ongoing)
* I've learned some English.
* I know some English.
More specific:
* I have studied English for [number] years/months.
* I have been learning English at [school/language center].
* I have a degree in English.
The best option will depend on the specific situation. For example, if you are talking to a potential employer, you might use the more formal options. If you are talking to a friend, you might use the more informal options.