Here are some more formal and natural ways to say it:
* "I have lunch." (Present tense, meaning you currently have lunch)
* "I ate lunch." (Past tense, meaning you already had lunch)
* "I got lunch to go." (More specific, meaning you bought lunch to take with you)
* "I had lunch with [someone]." (More context, meaning you had lunch with someone else)
The best way to say it depends on the specific context of your situation.