on the sims 2 for the PC, you need a female sim and a male sim (seems logical enough right?) that is unless you have a certain mod or hack that can have same gender pregnancies...anyway... the female...
The sims either have to be in love or married. Buy any double bed, (the bed where 2 people can sleep) have both the sims relax, have them cuddle, and you can then hit the button try for baby. If you...
Make them relax on a bed with the Sim who you want them to get pregnant with, then have them "try for baby" - if they are pregnant there will be a lullaby. Although having a pregnant vampire might...