Take your time! Pregnancy is a 9 month marathon, and giving birth is it's own marathon. Your body has been creating and generating life, and we shouldn't underestimate what that requires of us.
If you are nursing, eat well! I really felt like I was eating for two when I was breastfeeding my daughter, NOT during my pregnancy.
Get back into exercise as soon as you can, but very, very, very gently. Be kind to your body and do low-impact exercise. Also, depending on how your birth experience was, you may want to wait before doing any core exercise, but pelvic floor is extremely important in the months following giving birth. There are some great videos out there by pelvic floor experts, or you could get a physical therapist who specialises in this to help explain rebuilding the internal core muscles in a layered and gentle and deliberate way.
I found that my upper body strength was amazing having a little one because of all the lifting, but that my core and lower body were pretty poor, and my basic level of fitness struggled - because you're running on a total lack of sleep for a year or two!
All of that to say, prioritise your own wellness now that you are a mother, and I reckon it's worth seeking out some postpartum health and fitness experts.