This app was mentioned in 2 comments, with an average of 54.50 upvotes
To maximize your chances of successfully avoiding pregnancy while using the fertility awareness method, I would suggest that you research temping (taking your temperature in the morning before you move/talk/get out of bed) and checking your cervical fluid. Once you've kept track of those things for a few months, since your cycles are regular, you should have a clearer view of your cycle and your fertility.
Tracking your temperature will let you know when you've ovulated (your temperature rises after you ovulate) but on its own, temp tracking doesn't help you know when to start avoiding sperm. However, if your cycles are regular, you will probably ovulate around the same time each cycle, and you will also learn how long it is from ovulation to period start (called the luteal phase), which should also be the same number of days each cycle. Even if your cycles aren't regular, the luteal phase should be the same or very close to the same number of days, so you'll always know when to expect your period.
Tracking your cervical fluid, on the other hand, WILL help you know when to avoid sperm to avoid pregnancy. Your cervical fluid changes throughout your cycle, and you will get to learn how exactly it is when you are fertile, after tracking it for a few months. It's often described as being like raw egg white when you're fertile.
It's best to track both cervical fluid and temperature to get the clearest picture of your fertility, so you'll know what days you need to avoid sperm.
You will also need a different cycle tracker, one that focuses on achieving/avoiding pregnancy, that lets you keep track of cervical fluid and temperature. Two that work well for Android are Fertility Friend and OvuView. Both of these apps use multiple methods to help suggest to you your fertile and not fertile days.
You're probably getting downvotes simply because people have kneejerk reactions to "rhythm method". I think it's always a great thing for women (and their partners) to become acquainted with their fertility and cycle. Bodies are cool and weird and it's interesting to see how things work!
Just some extra information for you or anyone else reading this and considering using the fertility awareness method:
The typical use failure rate of the fertility awareness method of birth control is 24% (meaning 24% of women will experience an unintended pregnancy within the first year of typical use). Rates of combining methods aren't typically shown, but it's safe to assume that combining the fertility awareness method with withdrawal (all days, not just fertile days), as well as using condoms during fertile days will be somewhat more successful at avoiding pregnancy. Also, the more regular your cycles, the better this method will work for you. Since it's more likely you'll become pregnant using this method, it's a very good idea to have a plan about what you will do if you become pregnant. Hopefully, you can discuss this with your partner - being able to have a discussion about this sort of thing and being on the same page can help make your relationship stronger. :)
I've been using the following on Android:
During TTC Fertility Friend
During Pregnancy Pregnancy+
Once baby is born Baby Connect I can't say too much about this one as I haven't started using it yet, but here's the Website so you can do a little research :)