You need to get two sims to the partner stage, then you just max out their needs and choose the option to propose, choose One of the two most expensive rings, I advise you to wait until you get the goal to propose, the game then pays your ring. Then you raise the relationship to the highest level by doing romantic actions, then you can get married. Then you just buy a baby bed (not sure how it's called in english) from the baby menu. You can then add a baby to you town, profided you have enough room for another sim. Then you let one of your sims bake a birthdaycake, which you can buy from the supermarket, you can then use the option to let your baby have a birthday.
Hope this helps, any more questions, just send me a message.