We can't give you a percentage chance that the test is wrong, it's difficult to know such things. We always encourage taking a second test (usually a week later) to check the results.
Really sweetie, I'd encourage you to begin using condoms with your partner immediately unless you really want to get pregnant. Pulling out is NOT birth control. Pre-ejaculate, which contains sperm, is present throughout the course of an erection (even if your partner can't feel it or notice it, I promise it's still there). So way before he pulled out, there were lots of little sperm swimming away in there.
And about the STD thing, we always recommend not considering yourselves even moderately safe until you've been magomous for at least 6 months, using safer sex practices the entire time and have had two clean STD screens. And even then, understand that there is still a risk since some STDs are difficult to test for without symptoms being present, some STDs can crop up on their own (yeast infections, for instance), and some STDs can be passed non-sexually.
Go ahead and take another pregnancy test, or go to your doctor or local Planned Parenthood or other health care practicioner and have them give you a test. And then pick up some condoms and/or consider some other form of birth control. "Pulling out" is not birth control, it's playing with fire.
Scarleteen Sexpert (and Labia Lady)