Let’s talk ovulation tests

Do you start them directly following your period, If you don’t know your cycle? I’m about to end my first period since my MC, and before that, my periods were sporadic.... I didn’t get one until 15 months PP, and the first two were a joke, and my cycles were long. So I have no clue what my cycles are like. I’d love to make it work this cycle, but I’m really just wanting to use them to figure out when I ovulate and hopefully get pregnant within he next few cycles