NFP method

I'm interested in using the NPF method, but I wasn't sure what app to use or if there's a special thermometer I'm supposed to use. My husband and I will continue to use condoms every single time, but I am interested in learning more about my ovulation cycle because I'm completely lost and my period is all over the place. Sometimes my cycle is every 28 days and sometimes it's every 36 days. I had my son 15 months ago and I just weaned 2 weeks ago, so maybe that's throwing me off as well. What are you ladies using to track your cycles/ovulation?