Being accused of cheating on my python programming homework not sure what to tell the TA?

I started my homework on the day it was due very late after getting off work because I had forgot about it. I solved several of the programs and was having a hard time with one of the programs. I was using the internet to look up examples of how to do the problem. I was not looking for answers I was just looking for help in the form of a example which I could see how they did it and then make my code and solution doing it another way. Anyway I saw that someone in my class had asked a question on a website. I took a piece of the python code from there and was trying to figure out how it worked and to get it to run. I was then of course going to create my own code of course I don't believe in cheating. Anyway I couldn't figure out how it worked and so decided to go to the next question and come back to the code and the question. I was becoming very tired and frustrated when I noticed the time was almost up to submit my answers. I panicked figuring since I was almost falling asleep I should submit the homework I had done already and taking my lumps with the grade. Stupid me forgot to go over my code again and I left the code from that website on there on accident when i submitted the homework. The TA graded the homework giving me a 0 and asked to talk to me about my homework. I completely agree that I messed up and should receive punishment for this I don't believe though that I should get an F for the whole class though. My intention was just to submit my own work never anyone else's and I am deeply ashamed of what happened. I am just not sure what to tell the TA in this particular case because even though I am going to tell the truth I don't know if he would believe me. I am just wondering what to tell them. Also just wondering what you think should happen to me.