First of all, get a lawyer! Most lawyers will see you for an hour for free, go with a list of questions, don't wing it. Most of them will be able to tell you where to go if their fees are too high for you. You MUST get legal aide.
Second, keep a journal, take notes. Write things like...
"Jimmy picked baby up 3 hours late today, he screamed at me and left with her. He was 4 hours late returning her March 5th, 2008, she was dirty and had no bottle." "Talked to Jimmy today at 3pm for 15 minutes, he says he isn't paying a dime for child support and no court can tell him what to do."
Call CPS if he comes drunk or high. Call the police if he has the baby and you KNOW he is drunk or high and have them do a "well child" check at his house. GET IT DOCUMENTED! Does "Jimmy" have a record?
You say he is suing for sole custody, on what grounds? You say nothing about yourself. Do you have a job? A safe place to live for yourself and baby? Is your baby well fed? Up to date on her well baby check-ups and shots ($6.00 or so at the county health dept. you have NO excuse and a judge will say so) Do YOU have a record? Do you have a support system of family and friends that can help you? The safer, more normal your life is the more likely you are to have custody of your child. If these questions are NOT true then you need to start this moment and make sure that they are. The courts only look at the facts in front of them and they don't like hearsay, which means "well he said he would do...she didn't do..") Have it written down, date, time, who was there.
You are lucky in that you are the mom, not the dad. Mother's get custody MOST of the time. If you were in the state of Texas it is 96% of the time, I believe in Illinois it is closer to 88% of the time.
Yes, I know way more about this subject than I would like to. =(
Clean up your life, get a journal, get a lawyer.
Take a deep breath. You have more power and strength than you think you do. *HUG*