If a precum come inside me a day before ovulation, what are the chances of being pregnant? i'm 17 and still virgin.

Not zero!: The chance of pregnancy exists, what do the odds matter, if you did get pregnant? If this just happened (within 5 days from unprotected sex) consider Plan B. See http://ec.princeton.edu/pills/plan-b.html. You are definitely at risk for a sexually transmitted infection. Get checked. Practice safe sex and protect yourself from now on!