How To Determine Fertile Days Using Last Menstrual Cycle In Women With Regular Periods?

Hi my name is jasmine am trying to have a baby but I have in regular periods me and my husband are you what can help me I looked up if you are under weight are it can be costed by sickness am not sick no illness or anything so what can it be I will stay on for 4 days and be off for like a hole month and a couple days in the Second month until like the 7 to the 8 then I will come on do you have any advice to give to me or him