Because all of us have different reactions to various foods, there is probably no one way to eat badly or healthily when you are breastfeeding.
You will certainly need to maintain a good calorie intake and now is not the time to be worrying about starving yourself to get back your pre-pregnancy shape. However, there are some calories which are more nutritious than others, and a diet of cream cakes and crisps isn't going to provide the energy you need as a new mum.

Are you eating enough?
The great news during the breastfeeding months is that it's a great way to burn calories. However, if you aren't taking in enough, not only will you lose weight but your baby won't thrive as well as she could. Make sure you eat well and let your body and your baby's weight be a guide to a common sense approach to eating.
As your baby grows and your feeds reduce you will be able to take your consumption down a little, but do this carefully so you don't feel too rundown.

Foods to avoid
There is some argument for limiting dairy product intake as this can cause colic in some babies. Dairy-related food allergies are quite common and this can affect some babies, but if your baby is healthy and happy, dairy products are a good source of nutrients for you so don't elimiate them unnecessarily.
If your diet is usually full of garlic, onions and spices, your baby may well deal with them without any problems, but if your baby tends to be more windy after you've been eating these foods, try to reduce them in your own meals for now.
Caffeine in small amounts won't affect your baby, but larger amounts may affect her ability to settle. Try to have only one or two caffeine drinks a day.
If you have a family history (on either the mother's or father's side) of allergies, try to reduce your intake of any triggers in case they have affected your baby. If you have chest problems (asthma or asthma-like problems such as hay fever), try to limit wheat and dairy products or cut them out completely, BUT only after taking medical advice.
You can eat all those cheeses that were forbidden during pregnancy, but do so in moderation, especially with new babies. They are very rich and, whilst they shouldn't cause any health problems, they might result in a few very messy nappies!