Here's the IRS page on the Plug-In Electric Vehicle Credit
Here's the IRS web page where the IRS states that the Volt qualifies for a $7,500 tax credit:
Here's the IRS form you will fill out and file with your form 1040:
On lines 3 and 9 you will enter "$7,500". On line 11 you will enter the total tax you owe for 2010 (regardless of what you may have already had withheld from your paychecks). Then on line 12 you enter the total of following tax credits that you qualify for:
Foreign tax credit, Credit for child and dependent care expenses, Education credits, Retirement savings contributions credit.
On line 13 you subtract all those other credits from the tax you owe (regardless of what you may have already had withheld from your paychecks).
On line 14 you enter wither 7500, or you enter the total tax you owe after removing other credits (from line 13), whichever is smaller. So you only get to take the full $7,500 tax credit if you owe at least $7,500 in taxes after other credits are taken out (regardless of what you may have already had withheld from your paychecks).
Then you enter the $7,500 (or lower amount) from line 14, onto the credits section of form 1040, where it will be subtracted from the amount of tax you owe (regardless of what you may have already had withheld from your paychecks).
Here is the 2009 form 1040 (2010 is not out yet):