Formal:
* I only speak English.
* My native language is English.
* I am fluent in English but not other languages.
* English is the only language I speak.
Informal:
* I just speak English.
* I only know English.
* English is my only language.
* I'm an English speaker.
More specific:
* I speak English as my first language.
* I am a native English speaker.
* I am proficient in English, but not in any other languages.
The best way to say it depends on the context and who you are talking to.