#1. Go see a Doc like others stated.
#2. Report a claim to your Insurance Agent, they will give you advice or even handle the claim themselves and you just sit back and receive a check. You must however report the claim to your insurance company with-in 24-36 hours. Contact your insurance agent first, he will answer most of your questions.
#3. In 3 days you will have to travel to the police department to pick up the police report. Bring cash because they charge you 1-5 dollars for a copy.
#4.Contact Their insurance company if your insurance company decides not to follow up on the claim. Some don't when you do not have collision coverage on your car.
#5.When you contact the other parties insurance company (located on the police report by a numbered ID number, look it up on the net), you will be transfered to their claims office. They will ask for a police report. Then they'll mail you some forms to fill out that make you descibe what happen in your point of view. After they jerk you off for lets say... a month or two, you will receive a settlement from them. Most likely the value of your totaled car. You don't have to worry to much, since you have a witness, and he rear ended you which is the best place to get hit if you want 100% of your claim.
Hope you feel better!