If its only happening when you are aroused, its precum. Have you ever had sex or oral sex? If you haven't, then it can't be an std.
yeah ive had sex, and it happens when im thinking bout sex if i dont or do have an erection is that normal? i mean it seems like alot and when im horny but not erect, so do u know anyway to stop it?
If its normal, there's nothing to stop. If its happening at inappropriate times, maybe try directing your thoughts away from sex.
Its a normal thing for your body to do, though. If you are concerned, though, check things out with your doctor.