What is the best way to get pregnant when you don't get your period every month?

It depends! When you don't get a period every month, it means you have a hormone imbalance. There are many different types of hormone imbalances, and it's imperative to get to the bottom of which yours is, as each type can have certain medical and reproductive ramifications. See a board certified reproductive endocrinologist to get tested so he/she can most appropriately guide you thereafter.
Seek evaluation. Irregular periods usually indicates ovulatory dysfunction. Since there are many reasons what may bring on ovulatory dysfunction (pcos, hypothalamic amenorrhea, hyperprolacitnemia, perimenopause or poor ovarian reserve, hyperandrogenism, etc.) each with its unique treatment options to conceive, an evaluation is the most important first step.