You need potions called inseminals. You can start with them if you choose the right option. You can buy them from wicked Pam as well. You will also eventually unlock them as you level up your alchemy.
Maybe the best way though is to visit the fertility clinic in uptown. If you don’t have any inseminals in your inventory, you can ask the doctor at the fertility clinic to give you some.
After giving birth the first time, you’ll ublock traits that you can purchase that allow you to get pregnant without the potions.