If you're not eating a good, healthy diet and drinking plenty of water then your supply is likely to suffer so your lo will not get enough milk. I have found that I am constantly hungry when bf'ing, so I try to eat regularly throughout the day to keep me going. You need something in the region of 500 extra calories a day to bf, I believe, so you should be eating more than usual.
If you think you are suffering from low supply, there are ways to increase it - pumping after feeds, drinking plenty of water, taking fenugreek, eating oatmeal for example.