you need to get two sims married. to do this you first have to make them partners and max out that stage. Then you max out their needs and make one propose to the other. buy one of the two most expensive rings. If you wait for the goal to get them engaged, the game'll pay the ring. Then you max out that stage and you'll have the option to get married. Do this. Then buy a crib from the baby tab in the shop menu. If your two married sims live in the same house and the crib is in that house too, you should have an add sim option appear over the crib, tap this and you should be able to have a baby, if you wait for the goal to do this the game'll pay it for you, it costs a few lp so you might want to wait.