It depends entirely on how bad your separation is.
2-3 fingers (how they measure it) can usually be corrected with exercise. The way to start is with deep breathing and pulling your belly button back towards your spine. I would also recommend a splint for a few weeks to train your abs to go back together (I simply tied a scarf around my waist).
Planks will retrain your transverse, which is the key mucle to train post-pregnancy. Again, remember to pull your belly button in towards your spine as you do them. That motion will begin the repair.
DO NOT DO CRUNCHES, and go very easy on obliques until your split is healed. You can test this by laying flat on the floor and lifting up your head. Feel along your belly button and up towards your ribs. How many fingers you can get in there will answer that for you.
A good video to get, if you prefer that guidance, is "Lose your Mummy Tummy" by Julie Tupler. It deals with diastasis recti specifically.
All this said, if your split is severe, exercise may do nothing for you. My SIL had a 5 finger split after 2 kids, and required a full tummy tuck and ab reconstruction to get back to normal. No amount of exercise she could do would help. If you're looking in to surgery, don't bother to do it until you're done having kids.