If a girl masterbates prior to having sex the first time, will it reduce the pain during the frist time?

Introitus: Masturbation can increase the width of the introitus depending on the technique used; if it affects the opening, it may reduce discomfiture (but so will relaxation, ample lubrication, foreplay and a gentle, slow penetration approach; and, don't dismiss the endorphins released in the passion of the sexual encounter).

Not necessarily: The pain with 1st intercourse can be thought of 2 ways. First, the hymen may tear and it may hurt. Second, and perhaps more important, if you are tense and scared, the muscular tension may cause pain as well. So - if you masturbate and it helps relax you, i suppose it could reduce pain with intercourse. But if you are that tense, perhaps you are not ready yet for sexual activity. Use protection!