Trying to conceive. Have hypothyroidism, vitamin D and B12 deficiency. How to prepare for healthy conception?

Hi! I am 28yrs old my husband is 30 yrs old. We plan to make a baby now. We need to know how we can prepare ourselves for a healthy conception. How much time do we need to prepare ourselves. What all tests we need to get done(if any tests are req).
I am hypothyroid patient and i take 100mg medicine. Both of us were detected with vitD and B12 deficiency. My deficiency was high and my husband had a minor deficiency. My mensuration cycle is of 28days but discharge is scanty (for 2days). my husband has minor issue of sleep apnea. his father use C-PEP machine while sleeping. my husband has family history of diabetes & heart disease. my mother has diabetes and high BP too.kindly guide.