In the sims 1: You make two sims of the opposite sex kiss or "play in bed" and you'll get a question, to which you should answer yes.
In the sims 2: Make two sims of opposite gender (unless there's a same-sex hack) relax in bed, and with a high enough relationship, make them "try for baby". You should have a baby 75 hours later, in game time
In the sims 3: Pretty much the same, except you don't need to relx in bed first, you can do it directly from the menu.
In real life: Get a guy or girl and have intercourse with them
(Addenda, added by a different user)
And an alternate way to have babies, if you are thinking adding a baby to a household. Adopt: in Sims 3, use the phone and call the adoption services. MILK FOR THE UNYUUFEX, FLAT CHEST FOR THE CUTENESS THRONE, SKULLS FOR THE SKULL PROBES (user talk:Mathetesalexandrou) 04:16, March 13, 2012 (UTC)