The stomach area is indeed one of the hardest areas to work on. I myself struggled (and is still struggling) with losing the fat around my belly. One thing I have learned is that it is impossible to burn fat off from just one area of the body. So integrating cardio and core exercises with abdominal exercises will yield better results.
I think the best abdominal exercise for beginners are crunches, hip raises and planks. You can easily find free resources on abdominal exercises or routines online. Try doing a search in youtube. But if you want more structure, you can consider investing on Fit Yummy Mummy. Being a busy Mom myself, I find this program really helpful.