"I confirm that I am coming to the meeting." This is absolutely clear.

I can confirm that i will attend the meeting.

You've got to say: I confirm that I am coming to the meeting. If you say: I confirm that I will come to the meeting. It implies you had not planned that activity beforehand, but you've just decided to attend.

Adding to what Shaun said, also making it past tense (adding -ed to the word 'confirm') will also correct both sentences. "I confirmed that I am coming to the meeting." "I confirmed that I will come to the meeting."

I would recommend adding "can" to the sentence, as below: I can confirm that I am coming to the meeting I can_confirm that I will come to the meeting I can confirm I will attend the meeting