Betty, the credit is not automatically $8,000, it is UP TO $8,000, depending on your income level. The best advice is to tell you to get with a trusted local Realtor who can explain the credit to you in detail, and can represent you as a buyer's agent. You'll want someone looking after your interests! Good luck!
Could you clarify? Is it that you don't actually pay income taxes or don't owe any? Either way, to get the tax credit you have to file and include the Fed Form 5405. The $8000 is netted against what you owe or are owed back. If you owe $0, then you get $8000. If you owe $1000 then you get $7000. If you get a refund, then the $8000 is added to the amount you are owed.
Either way, it is to your advantage.