For more serious, whole-body exercises, most birth professionals advise you to wait a little longer. Speak to any gynecologist/obstetrician, and you will probably hear that you should wait for at least six weeks before commencing exercises. By this time, most women will have noticed their lochia (postpartum bleeding) come to a halt, and will generally feel much more like themselves again.
Six weeks postpartum is also the normal scheduled time for your post-birth medical checkup, so it is a convenient time for you to talk to your doctor or midwife about your body and whether you can exercise. Still, if you feel up to exercising before six weeks are up, and you can somehow find them time, there are safe work-outs you can engage in. A little stretching or gentle yoga is certainly fine, and walking is another exercise you can do together with your baby, that is fine as soon as you feel up to it.
Listen to your body and make sure to stop when your body tells you to stop, though. That is key to healthy postpartum exercise. And also make sure you don't do ab crunches or other abdominal exercises too early. Your abdomen needs to recover, and its muscles need to move back together after you give birth. If you do ab exercises before this happens, you risk permanent diastasis recti.
Sexual intercourse should generally be delayed until you feel better as well, just like exercise. Don't feel afraid to discuss this with your doctor or midwife either, as doing the deed postpartum is perhaps an even more daunting adventure than weight loss after pregnancy. The first time, that is it will get better, we promise!