The official site is
CARS.gov - Car Allowance Rebate System - Home - Formerly Referred to as “Cash for Clunkers”
To find out what cars qualify, go to
Fuel Economy and search for your year and make and model. I know for regular cars (not trucks), the combined mpg must be 18 mpg or less to qualify for this program, I'm not sure what trucks qualify, but the first site should let you know if you have a truck to trade in.
Check first to see if your used car qualifies. Then you will need to find an eligible new car, as you can not buy just "any" car with this program. For example, if your used car gets 18 mpg, you will need to buy a new car that gets at least 22 mpg combined to get $3500 for your used car. If you buy a new car that gets at least 28 mpg combined, you will get $4500 for your used car under the program.
If your car is worth more than $3500 or $4500, then when you go to the dealer, you don't want to use the Cash for Clunkers program and you should just trade it in as you would a normal car. If your used car is eligible and worth less than $3500 or $4500, then you will be getting a better deal than if you were just trading in your car.