Written reprimand at work for fed government workers?

I used company letterhead for unoffficial business. Some information wasnt exactly accurate. I have to meet with mgmt tomorrow. what do i say in this mtg. tell all? i spoke briefly with tmy director first i heard they knew. i was wondering why i wasnt receiving my outstanding on my performance when my supervisor was recommending highly, he told me how can i when this is happenening. he said not to worry but he is not involved in the mtg tomorrow? what usually happens? should i talk and incriminate myself be honest. I dont know what is the 1st step they do??