I've seen this word misspelled so many times lately, so let's get to work. We’ll start with a definition because I think that will help you remember how to spell it:
DEFINITE or DEFINITELY = limits or finite quality
The etymology (the history of a word) shows that it comes from the word FINITE, which will also help you remember how to spell it.
Since you know that FINITE is spelled with an I and not an A, you should have no problems remembering, now, that DEFINTELY is definitely not spelled DEFINATELY.