repercussions of positive drug test

I am a 48 year old A&P mechanic with almost 30 years of flawless service. I work full time for a major carrier, and part time for a local freight company. On Wednesday night I found myself with some freinds and I shared a joint (marajuana) I don't know why, but I did. 14 hours later my part time job popped me with a drug test. The results are not back yet, but I am a wreak. Surely it will show up.

I cannot even be described as a casual user of drugs, this just happened. Obviously my part time employer will terminate me.

What happens with the FAA? Will my full time employer find out? What can I do right now to save my full time job? Turn myself in? Enter a program? Do nothing? Please help.