Ok...
You are seven. The average life expectancy for people born in 2000 is seventy-seven. So you still have seventy years to live, provided you take care of yourself (eat healthy, don't smoke and don't do drugs). You don't need to be scared. Unless you're talking about that because you read some chain letter that said you will die in 3 days or something, and if that's the case, those things are all made up. People just make them for the same reason someone would make a virus, to be annoying. Anything it said is totally bogus. You shouldn't be worried. You've got your whole life ahead of you, just don't take drugs, smoke or eat bad food.
And, by the way, you are not going to be hit by a car or have a nuclear bomb dropped on you =P
You've got nothing to be afraid of.