When is the right time to file a claim for personal injury?

My friends and I were recently hit by a driver running a red light. It's been 4 days, and since then, I've been to the ER twice. The first time after the accident, the second the other night after my doctor ordered a CAT scan, and I had an allergic reaction to the contrast dye. I have been feeling fatigued, my arms hurt, I am nateous, not eating and sleeping well, my lower back hurts, and I can't concentrate as well as I normally do, like I am actually using spell checker now! Do I actually have a claim against the guy who hit us, should we file all together, because my 2 other friends also have whiplash injuries, and do we even have a claim? How do we even get the process started? Please help if you can, I am feeling kinda helpless here...