Irregular period???

How do you figure out how to find your fertile window when you have irregular periods? I don't use this much but I hope it will help. Trying for baby #2!! I don't know how to track bbt. Can someone explain for me? According to the data I've entered on here so far, I should ovulate from the 28th to the 3rd. How do I know if that's right??? And what ovulation tests are the best to use?? Sorry for so many questions!