You don’t need anything at all before contacting a lawyer.
That said, the more information you have about the Who, What, When, and Where of the accident, the better. How did the accident occur? Who was at fault? Did you see if and how they were distracted? Did they admit it was their fault at the scene? Was it a multi-car collision? Who witnessed the accident? Any information you have on your injuries or the medical attention you’ve received is also helpful.
If the accident was a hit and run (and you’re the one who was hit), you won’t have much more information than the make, model, and license plate number of the car – if you’re lucky. In that case, you may want to consult your insurance company about Uninsured Motorist coverage.