I brought a pregnacy test and I open it and it had a faint negative already on it should I still use it even tho its brand new?

No. I would take it back to the store and exchange it for another one. The area where you read the test as "positive" or "negative" should be completely blank prior to you using it. If there is already a marking on it, then i would assume that particular test is defective and wouldn't trust the result it gives me.