So someone has been using my SSN for the last year or so.. what can I do now?

I'm not exactly sure how to approach this, I'm 20, still in college and have no clue on what to do.

A few years ago when I was still a teen, I carried my wallet out every day and I had a piece of paper in there that had my SSN on it (I know I'm a dumb POS for this). I noticed one day that it was missing and I realized the only thing that could have happened was when I pulled out my wallet to buy something and I must have dropped it somewhere. I panic for a few days and think nothing of it. I'm not exactly sure this is even where it starts, but it's the only way I can think of how my SSN could have been leaked. Recently, my parents have been talking with the insurance/medical/membership guy at my local hospital and she called me back saying I couldn't get approved for their program because I was above the low income level (combined with my parents). She reported that I've been working last year and made over 5k under my SSN. I havn't worked since my high school internship 3 years ago in 2008. Recently, I've noticed I've been getting a lot of these mails for credit cards like american express with my name spelt wrong or in a weird order. I checked my credit report and noticed there was a date on it 05/05/2010 I do not know the significance of this date, but I do know that I've never gone to this annualcreditreport website (until today to check) and did not file anything on that date. My address is strange too on the site, it's a little subtle but I never write it like that. [Person spelled it as Av. rather than what I usually do Ave.] I checked this site as to see if any cards or bank accounts have been opened under my name. All I know now is that my name was spelled wrong, and it seems there isn't anything besides my own accounts and cards. What the lady told me was that this person was working last year and made over 5k and that it is in a restaurant in a nearby city.

The thing that worries me the most is that even if I do stop this and save myself for this year, I'm afraid it might turn up for other things. This person only used it for working, I assume, he/she is either an illegal immigrant or someone else who doesn't have a work visa. The restaurant, in turn, I don't know how they could let something like this slide, but I'm pretty sure they would know too. The "other things" I'm talking about are things like financial aid, medical insurance (right now, I guess, but only going to delay me), and I guess anything else. What's really stressing me is that this person can use this number (for other things) and will always know this number for as long as I live. Maybe I'm overworrying, maybe I'm not worrying enough, but right now, I really just don't know what to do with this.

TL;DR Person has been using my ssn for the past year (maybe even two/three). It is affecting my insurance renewal and possibly other things. Noob redditor OP doesn't know what to do.