Need tips on trying to conceive.. pre conception prenatal?

Im 19 , I've been using this app 3 months now && haven't had any hope .. I always go by my calendar on wen me and my partner should have sex

But then wen it time I either have a long cycle or a very late period. It's been stressful cause we both want a baby ❤ any tips on what I need to do ? Cut back on ? Buy ? Etc .