I Received A Subpoena Because I Witnessed A Dwi Accident...

I received a subpoena in the mail today from my county. In August of last year, I witnessed a single car accident and the driver ended up being charged with DWI. Not immediately knowing the circumstances, I called 911 to report what I had seen. A local LEO soon called me back and asked me to describe what I saw, which I did. Fast forward to today, I get the subpoena in the mail. The accident happend in my hometown, so it will be in my local court. Appearance date is exactly six months to the day since the accident occured. What I saw was fairly straightforward, even though it was months ago, details are not hard to remember, times, etc. Any advice on what or what not to say, bring, etc would be appreciated. I have never been in a situation like this before, so I am kind of nervous. I don't want to ruin someone's life, but at the same time I feel what I did was right.