I had a baby 3 weeks ago, and I was wondering when I can start doing sit ups to get rid of this belly I have?

Just conentrate on taking care of your baby, the belly will take care of itself after a couple of months, just be patient. You will lose your 'baby fat' if you are nursing, if not, then it will take longer. Watch your diet but don't deprive yourself of nutrients your breast milk needs. When your little one gets older, maybe a 2 or 3 months, there are exercises you can do with her/he to help those abs get back in shape. It is all fun, just enjoy it.

At least wait till your first check up and then ask your Doctors advice on how soon would be safe. Enjoy your baby right now, your body needs to heal a little before you start exercising.