With the sentense "I'm having breakfast",
If you mean "I'm having breakfast right now", it should be "Watashi wa ima asagohan wo tabeteirutokorodesu".
If you mean "I'm about to have breakfast", it should be "Watashi wa asagohan wo taberutokorodesu".
If you mean "( I may not be able to have lunch, so I should/will have breakfast. And) I'm having breakfast", it should be "asagohan wa tabemasu".
Please note that "ha" is "は", the pronounciation is "wa".