Yes, you need a criminal defense attorney. You are doing the right thing by trying to stay on top of this. The trouble you got into out of state may matter or it may not matter - it depends on what it was, how long ago, how old you were, what the resolution of the charge was, etc. When you meet with an attorney they will be able to give you a more precise answer.
Attorneys on AVVO are not supposed to make referrals to specific attorneys on the AVVO answers forum, so you will need to make some phone calls. I would start calling people right away so you can be sure to have time to consult with them and have them retained prior to your arraignment date.
I agree you need to find a lawyer. But you also can calm down a bit. The subpoena only brings you to court for arraignment, which is a formal reading of the charge. You enter a plea of not guilty then and get another court date. Your lawyer can then get copies of the police reports and get more details of what the state believes occurred. Reviewing that report with your lawyer may help you remember more details of what happened. Consulting with your lawyer, you can explore avenues of investigation and see what witnesses might be available to help you. You don't have to have all the answers before the arraignment. You will have time to work through what your options are and what you need to do before the second or a later hearing.
It is a good idea to retain counsel before the arraignment so she or he can help schedule the next hearing and get the police reports as soon as possible.