My daughter's father and I have not been together for a year now and at first we were keeping her every other week and splitting everything. Then I started struggling so he kept her for a couple months while I got on my feet.Now I have had her for the last 2 months straight and he sees her every now and then.He's been helping me because I don't have a job right now with the economy but I'm looking hard... I am sick of him holding everything over my head and I'm looking to file for custody and child support but I'm worried I'm not going to get it... He's a sexual offender, does that carry any weight and because I'm the mother will I more than likely get custody of my daughter?

Yes you need to get a court order so you have the order to back you up then he has nothing to say. Main thing at this point is make sure you can prove you are active in her life. If he could go into court and say he has had her and you haven't been involved then you would have worries.
I am not sure if his being a sex offender would have much weight in court since they will see it as you have been okay with it up until now. They may look at it as you grasping for straws.
Make sure she is staying with you and you can tell the court you have had her with you for the past xx months. That will be about the best plus for your side.

You will likely get primary phsyical custody but joint legal custody. He will get visitation and be required to pay support.

What is his sexual offense? If its against children he may not get any visitation!