How do I negotiate starting pay AFTER accepting it?

I just accepted a position with a staffing agency and the position starts with $12.00 (based on previous position pay). She asked if it was ok, and wrapped up in the excitement of getting a job (been out for months) I said it was fine. I really wanted to start with $13.00. Being that I actually work for the temp service and not the actual company (it's a 3 month probation period before I'm hired on permenately with company) is it too late to ask for more than $12.00 since I already said $12.00 was ok? If not, how do I ask about it?

Well, it won't be easy now.
How needed are you? Could they get along easily without you or are you picking up too much of their slack?
Is your job involving a unique skill, where special training is needed? Are their others at the temp agency waiting to take your place?
Consider these questions first. The raise you are talking about is very minimal so saying you've offered something better somewhere else seems a little ridiculous. If after considering the above questions, and feeling you want to push your luck a little, tell your employer that dollar an hour would really make a difference for you, paying the bills, etc. If they disagree, they may feel obliged to offer you more than an extra dollar. Otherwise, they'll just say no and that will be the end of it. Don't seem greedy, and apologize for not speaking up earlier. But for all intends and purposes, asking may be worth it.

The boat kinda already sailed on that trip.

It is a little late. You should work there a little while then ask for more. Otherwise you would have a negative perception.