My question involves criminal law for the state of: Wisconsin

I was pulled over about a day ago in Wisconsin for speeding.. When the xop pulled me over he said he smelt marijuana coming from my car. Well I was coming from work and wasn't smoking at all. He told me to step out of the car. I told him I had nothing on me and he asked to search my car I said I didn't see the reason and he threatened to bring the k9 unit and tow my car. I then told him that there was a small amount in my overnight bag. It was literaly a pinch in a pill bottle from the weekend that I never took took of my car. I have never even bought weed before. It was a tiny bit someone found and gave to me. I know it doesn't make a difference it was just kind of crappy how it all went down. Reguardless I was cuffed and arrested. They finger printed me and took my mug shots. I got a possesion of thc ticket as well as a speeding ticket. The possession of thc was for $429. The officer told me to call him if I had any questions. I did but he still hasn't gotten back to me so this is why I'm on here. I was also looking at talking to an attorney. I have no idea what's going to happen to me. If I plead guilty will I go to jail? He told me my job wouldn't find out and neither wouldy college. I ha two Friends that got busted in the dorms in college and they just had to go through a class and I think it was taken off thier records. Is there any way to get it expunged? I don't want it in the newspaper or to show up if someone does a backround check on me.... This is my first offense and definately my last. I am 19 years old. What I basically want to know is what happens if I plead guilty or is there anything I can do to get it exspunged. I have no idea what's going on and I don't want to go to jail. Any help is greatly appreciated.