What should i do to prevent pregnancy?

Patient: I had a baby on 4 december,2011. Now my baby is 2 months old, in between we had intercourse n I went to my lady docter but she told me to wait for the next date, I’m afraid that i’ll b pregnant if i’ll wait for my menstruation date, what should I do to prevent pregnancy now? Can u plzzz help me?i’m feeding my baby too, Plzz reply as soon as possible, thanks.