Never appear in criminal court as a defendant without a lawyer. What happened sounds like embezzlement but even if it is simple theft you need a lawyer to avoid screwing up your life. If you can't afford one, go with the Public Defender. Even though many people complain about them they are good lawyers and good people.
Note that I am licensed to practice in CA not Washington state; this is simply general educational advice. To get a definitive answer get a WA attorney.
SEE A LAWYER. Right now. Stop writing about this and don't talk to anyone other than your lawyer about it. What you just wrote, if it can be traced to you, is what is called an admission and could be introduced in evidence against you. What you did is a crime and could be punished, even for a first offender, by jail time. If you cannot afford a lawyer, go to see you local Legal Aid Society or Public Defender's Office.
On the bright side, as a young first offender, it is quite likely the system will go easy on you. In most counties in my home jurisdiction (New York), you would probably get an ACD (adjournment in contemplation of dismissal) which would mean you could have a clean record if you stayed out of trouble for a set period of time.
It does not sound to me like you have either a defense or a lawsuit against the company, although reasonable lawyer can differ in their interpretation of the facts. In terms of a defense, the store detectives are not what are called "state actors", that is they are not officers of the state, but of a private organization, so most of the protections you see on TV (Miranda warnings, etc) just don't apply. They may have misled you, but they did nothing else which would implicate a protected right. It may also be they told you the trutch and only decided to call the police after you made a confession. In most jurisdictions, an 18-year old is an adult for all purposes - neither the police nor (particularly) company security needs to call a parent, even if requested (unless the parent is the lawyer for the accused).
In terms of a civil suit, it is a bit difficult to say in substance: I was a thief, they caught me, I confessed and now I'm entitled to something because they lied to me about whether I was going to be charged. To me, that is a non-starter. As I said, reasonable lawyers can differ and you may want to discuss this with an attorney in person, but I wouldn't count on a success there.
I am not your lawyer and you are not my client. Thus, you should not depend on the foregoing as attorney-client legal advise. I am sure there are things you will discuss with your lawyer that you have not detailed here, and there are certainly things your lawyer will want to know and ask you about. But the bottom line is: you need a lawyer - see someone as soon as you can. Don't go to Court without legal representation and don't talk to anyone other than your lawyer about this situation.
The prior answers were right on, you need to talk to a lawyer licensed in Washington state as soon as possible and to not talk about this with anyone else or publish any details on websites. There are many possible consequences and you need someone who can provide you individual advice.