You should contact an attorney as soon as you are mentally and physically able to. Valuable evidence is lost with each day of time passed.
Often times, in complicated car wrecks, or in uncomplicated car wrecks, there will need to be a reconstruction in order to prove fault. In order to do that, you need to get someone out to the scene to start taking photographs and measurements as soon as possible. A law firm such as mine has contacts within the accident reconstruction industry, and we can get on that immediately.