Most obstetricians and gynecologists advise postpartum mothers to wait for at least six months before they try for another baby. For mothers who gave birth via a cesarean the advice varies, though 18 months between once c-section and the next seems to be recommended often. Women who are breastfeeding on demand and not supplementing with formula often don't get their cycles back for many months (and in my case, even years!) after they gave birth. In that case, your body will practically make the decision for you, and you will not be able to conceive again until ovulation comes back (use ovulation tests if you are not sure whether you are ovulating yet).
There are, of course, women who find out that they are pregnant again very soon after the birth of their baby in some cases, they can conceive within a month after the arrival of their baby! While this is not something that any doctor would recommend their patients actually plan, you will most likely be just fine if this happens to you. Your pregnancy might well be harder on your body, not in the least because you have a very young baby to take care of. But having two siblings very close together also has its benefits. You might also take a look at trying to conceive after an ectopic pregnancy.