First of all you should find the cause that you want to contribute for, and thus select the charity that is working for that cause. Make sure it is a registered charity as otherwise your claim for tax benefits may not be entertained. Also, if the charity is not registered the charity itself cannot claim the form against the car value.
Determine Car Value
You can search online and look into directories to determine the market value of your car at the time you are donating it to the charity. This will also take into account the car condition itself. Also, you will need to deduct the amount of the total car value that needs to be spent on the car repairs etc.
It is better if you get a written appraisal from car dealer if you are claiming the value of the car more than $5000. This can help you in filing the tax benefit claim. The deal can also be able to tell you the market value of your car and since it will be in written it will make your claim even stronger.
Filling Out Tax Form
If you are donating car in the US and the car value is higher than $5000, fill out sections A and B of the 8283 form. Make sure you give all details in the form and answer all questions. Leaving any space blank can affect processing of your claim, or it might be rejected.
Do not send form without charity signatures. You can ask the charity manager to sign the section B for you. Also, attach a copy of receipt of acceptance of the donation by the charity. Since this will have the complete address of the charity, the tax department can contact them if they want.